PP23C-1480
Tropical Pacific Nutrient Dynamics From the Late Quaternary to the Modern Ocean
New measurements of the nitrogen isotopic composition of nitrate in the modern tropical Pacific illustrate the power of the d15N tracer to resolve aspects of upper ocean dynamics such as water mass origin, mixing, nutrient cycling, and productivity. Here we couple these modern day measurements with a network of nitrogen isotopic records from sediment cores across the equatorial Pacific to provide, in effect, a synoptic view of eastern tropical Pacific productivity and nutrient cycling over the ice ages. Specifically, the suite of sediment cores spanning the large gradient in equatorial Pacific surface properties is crucial to distinguishing between secular changes in the isotopic composition of nitrate and varying nitrate uptake by phytoplankton. The sediment d15N record from the nitrate-free west Pacific warm pool provides us with the record of secular nitrate isotopic variability for the tropical Pacific—a record that must be a product of either denitrification in the eastern tropical Pacific or pre-formed high latitude influences. This western equatorial Pacific sediment d15N record can then be used as a baseline to remove the secular d15N trend from sediment records in the nitrate-rich eastern equatorial Pacific. With this approach, we resolve nearly complete relative nitrate utilization for surface waters of the eastern tropical Pacific at glacial maxima. Variability in relative nitrate utilization off the Peru Margin is high during interglacial and low during glacial periods with a strong 100 kyr frequency.
PP23C-1481
A mid-Holocene transition in the nitrogen dynamics of the western equatorial Pacific: Evidence of a deepening thermocline?
Sedimentary δ15N records from the oligotrophic western equatorial Pacific (WEP) off Mindanao show that late Holocene sedimentary δ15N is substantially lower than that of the early Holocene, following a gradual >3 permil decrease that occurred between 7 and 3 kyrs ago. Analyses of modern day nitrate isotope profiles from the same region indicate the sensitivity of the WEP N pools towards 1) the advection of 15N-enriched nitrate from the Eastern Equatorial Pacific (EEP) by the North Equatorial Current (NEC) and the Mindanao Current in subsurface waters and, (2) at shallow depths, the input of new and 15N-depleted nitrate through N2 fixation. We suggest that the Holocene decrease in sedimentary δ15N reflects a diminished relative input of 15N-enriched nitrate to the surface biota, either through an increase of regional nitrogen fixation, a change in nitrate consumption along the advective path of nitrate supply, or a decrease in the vertical supply of 15N-enriched nitrate from the NEC. The latter mechanism is consistent with a Holocene deepening of the WEP nitracline/thermocline.
PP23C-1482
Comparing changes in sea surface and thermocline temperatures between the western and eastern equatorial Pacific since the Last Glacial Maximum
We present sea surface and thermocline temperature reconstructions from two cores in the eastern and two from the western equatorial Pacific (WEP) based on alkenone undersaturation and planktonic foraminiferal Mg/Ca spanning the last 30,000 years. Whereas equatorial Pacific surface waters warmed by about 3 degrees C during the deglaciation, foraminiferal Mg/Ca records from the two WEP cores suggest a steady increase in thermocline temperatures by about 10 degrees C between the Last Glacial Maximum (LGM) and today. These subsurface temperature estimates are based on Mg/Ca records from both Pulleniatina obliquiloculata and two size groups of Neogloboquadrina dutertrei (250-350um and 400-500um), which all yield very similar temperature estimates. In sharp contrast, Mg/Ca data from the larger specimens of N. dutertrei (400-500um) in the eastern equatorial Pacific (EEP) cores suggest almost unchanged habitat (thermocline) temperatures between the LGM and modern with maximal habitat (thermocline) temperatures during the deglaciation. Mg/Ca records from the smaller (250-350um) N. dutertrei specimens, on the other hand, show a steady increase in EEP subsurface temperatures by about 4 degrees. These results suggest that there may be two distinct populations of N. dutertrei in the EEP distinguished in part by shell Mg/Ca content and test size. We will compare our temperature estimates to species-specific isotope records, and discuss oceanographic implications of our results.
PP23C-1483
Comparison of Terminations I and II in the Eastern Tropical Pacific: A Cross-Equatorial View
The region between the equatorial Pacific cold tongue and the warm water pool offshore Central America to its north is unique within the tropical oceans for its strong frontal zone, dynamic character and high-amplitude variability on seasonal and interannual timescales. The long-term dynamics of this region, driven by coupled ocean-atmosphere interactions relevant to Pacific-wide climate, may be key for understanding tropical Pacific processes on glacial-interglacial timescales. We present new constraints on the dynamics of this frontal region from two cores located on opposite hydrographic ends of the front: V28-134 in the northeastern warm pool offshore Costa Rica, and V19-28 in the upwelling zone south of the equator. G. ruber (white) oxygen isotopes and Mg/Ca temperature reconstructions have been developed in both sites spanning the last two glacial cycles. We focus here on the pattern of deglaciation during the last two terminations with emphasis on (1) amplitude and timing of surface warming and (2) modulation of the cross-equatorial temperature gradient. Terminations I and II look significantly different in both sites. Warming during termination I amounts to approximately 2.5 K with a somewhat larger amplitude in both Mg/Ca and δ18O in the northern site, supporting a strengthening of the cross-equatorial front over the termination. During termination II Mg/Ca warming in both sites exceeds 4 K and culminates in a short-lived peak, which quickly decays to more stable interglacial temperatures. Unlike termination I there is no evidence for a significant change in the cross-equatorial front during termination II. Changes in G. ruber δ18O and Mg/Ca temperature at the onset of each termination are nearly perfectly synchronous in each site, with no detectable temperature lead. Implications of these results for the role of tropical Pacific dynamics on glacial- interglacial cycles will be discussed.
PP23C-1484
Tropics-to-High latitude tele-connections during past rapid climate changes: the seasonality paradox
ODP Site 1240 from the Eastern Equatorial Pacific (EEP) provides a unique archive to explore, at high resolution, the role of low latitude oceanographic systems in past global climate changes. A combination of both inorganic and organic geochemical proxies is applied to reconstruct temperature and salinity changes in surface and thermocline layers. The Sea Surface Temperature (SST) record based on G. ruber Mg/Ca ratios supports a surface warming during times of rapid cooling in the North Atlantic associated to the Heinrich Events. These results are consistent with speleothems records from Brazil and coupled climate model reconstructions, and support that diminished North Atlantic Meridional Overturning Circulation (AMOC) induced a southward displacement of the Inter-Tropical Convergence Zone (ITZC) and hence a weakening of the south-east trade winds. Nevertheless, a SST multi-proxy comparison illustrates a more complex pattern with enhanced seasonality during the last deglaciation induced by both orbital and AMOC forcings interacting at this time. Thermocline proxies support that combined low precession and high obliquity conditions lead to the establishment of "La Niña-like" conditions in the EEP that alternated with diminished upwelling conditions at annual or interannual time scale which apparently resembled intense "El Niño-like" conditions.
PP23C-1485
A Common Mechanism of Multi-timescale Abrupt Global Change
The La Nina phase of the El Nino/Southern Oscillation (ENSO) is known to cause global cooling on inter-
annual timescales through changes in deep convection patterns and reduced supply of water vapor to the
tropical atmosphere. Two distinct means are presented here by which this mechanism may also act on
timescales exceeding 100,000 years. Firstly, the hypothesis of millennial tidal forcing is revisited with the view
that equatorial buoyancy frequencies and steep internal waves in the Pacific Equatorial Undercurrent make
vertical mixing in the equatorial Pacific uniquely susceptible to incremental changes in tidal energy. Hourly
Tropical Ocean Array subsurface temperature data show a resonant response to extreme tides associated
with the 1997 and 2000 ENSO events. Complimenting the known 1,800 year peak tide cycle, a 550-600 year
cycle of three-fold variation in the frequency of deep central eclipses (gamma < 0.05) is consistent with the
timing of the Little Ice Age. Fortnightly eclipse triples (FET's) associated with this eclipse cycle are shown to
coincide with both warm and cold phase Southern Oscillation Index (SOI) inflection points between 1876 and
2007, and notably the cold phase maxima of 1904 and 1917. In the second proposed trigger, southward
migration of the intertropical convergence zone (ITCZ) in the central and eastern Pacific may periodically shift
the rising branch of the Hadley circulation over the equatorial cold tongue. The resulting winter monsoon
system develops an equatorially symmetric La Nina (ESLN) mode through a positive feedback between
diverging surface winds and meridional rather than zonal SST gradients. Exchange of latent heat in the winter
monsoon contracts the Hadley Cell, draws circumpolar westerly winds equatorward, and expands high
latitude ice volume, as demonstrated in 1998. A three million year record of obliquity and August 10°N minus
10°S insolation (AUG10N-S) shows an ice volume dependence upon the mutual direction of change of these
signals (rather than upon their quantity). This suggests an orbitally driven north-south ITCZ oscillator in which
increasing August insolation at 10°N steepens the cross-equator meridional temperature gradient and
strengthens the annual cycle when damped by southern hemisphere thermal inertia, and vice-versa.
Increasing Aug10N-S is shown to constrain rapid ice loss (ESLN off). Conversely, declining AUG10N-S
coupled with declining obliquity less than 23.5° triggered or maintained glaciation in 44 of 49 cases (ESLN
on). The above tidal forcing means may additionally act on precessional timescales because the FET cycle
has a seasonal maximum at aphelion, with a possible greatest effect when combined with equinoctial tides.
Also, the lunar day at new moons is shorter (closer to 24 hours) at each equinox, thereby extending periods
of luni-solar resonance at those peak tides. Tidal forcing may vary further with 100,000 and 400,000 year
eccentricity cycles, both directly and by perturbation of the Moon's orbit, and through possible secular
changes in the Saros cycle. It is proposed that an equatorially symmetric ITCZ is the necessary condition for
a cold phase response to tidal forcing.
http://www.johnduke.com
PP23C-1486
Divergent trends in Holocene SSTs derived from alkenones and Mg/Ca in the Equatorial Pacific
Holocene SST records from the eastern and western Equatorial Pacific that are derived from alkenones usually indicate a warming trend comparable in magnitude to the deglacial warming signal. Conversely, Equatorial Pacific SST records derived from Mg/Ca ratios in the planktonic foraminifera Globigerinoides ruber often indicate a cooling trend during the Holocene. We propose that the different temperature histories recorded by these two SST proxies result from the precession half cycle during the Holocene and the seasonal preference of alkenone-producing coccolithophorids and G. ruber. Specifically, although the data are limited, alkenone-producing coccolithophorids in the tropical Pacific appear to be most abundant in winter and G. ruber appear to be most abundant in summer. Furthermore, a complete half-cycle of orbital precession from 11-1 ka caused an increase in boreal winter isolation and a decrease in boreal summer insolation on the equator. If those insolation changes resulted in SST changes of the same sign then alkenone SSTs should have increased and G. ruber-Mg/Ca SSTs should have decreased during the Holocene, consistent with the observed trends.
PP23C-1487
Contributions From Compound-specific Radiocarbon and Size Fraction-specific Th-230 Excess Data Towards Understanding of Sediment Redistribution Processes in the Panama Basin
Despite considerable efforts in the recent years, the sedimentation processes in the Eastern Equatorial Pacific, namely the Panama Basin, remain the subject of considerable uncertainty. In particular, the occurrence of lateral sediment redistribution as suggested by constant flux proxies like Th-230 excess is the subject of much debate. Th-230 excess measurements performed on bulk samples imply strong sediment focusing, leading to up to 8-fold higher accumulation than vertical flux rates. Arguments have been put forward suggesting that the assumptions behind the Th-230 excess method may not be entirely valid. In several other regions of the world, comparison of compound-specific radiocarbon ages of marine algal biomarkers with those of co-occurring coarse-grained sediment constituents like planktonic foraminifera have provided independent evidence for the occurrence of lateral supply of allochthonous, pre-aged sediment. The interpretation of those data is based on the assumption that organic biomarkers primarily reside in the fine-grained sediment fraction, which is more susceptible to resuspension and lateral transport processes than foraminifera. Here we present results from a study combining both methods to assess lateral sediment supply. Using samples from sediment cores Y69-71P and ME0005-24JC from the SW Panama Basin, we performed compound-specific radiocarbon analyses of marine phytoplankton-derived organic biomarkers (alkenones) and co-occurring planktonic foraminifera. In addition, we examined the Th-230 excess distribution in individual sediment grain size fractions of selected radiocarbon-dated horizons in both cores, testing the hypothesis that laterally advected fine-grained material contributes more strongly to the total inventory of Th-230 excess than coarse-grained material. We found higher Th-230 inventories in the smaller grain size fractions than in coarse silt and sand sized material. Down-core variations in grain-size distribution may thus partly explain the observed Th-230 excess inventory. In contrast, radiocarbon data indicate no supply of pre-aged organic matter.
PP23C-1488
230Th in the Eastern Equatorial Pacific Panama Basin
In the Panama Basin, higher glacial sediment mass accumulation rates (MARs), determined using oxygen isotope stratigraphy, have been suggested to be due to high surface water primary productivity [1]. More recently, however, the 230Th normalization method suggests that higher glacial sediment fluxes are due to sediment focusing by deep-ocean currents (e.g., [2]). Moreover, the 230Th inventory suggests that lateral advection of marine sediment in the Panama Basin is 2-4 times greater than the vertical flux, provided that the sediment is unfractionated by lateral transport. Differentiating between these two models of MARs is crucial to our understanding of the response of sedimentation to changes in climate, and how sediments record paleoceanographic information. Assuming that the 230Th model is correct, downslope transport of sediment from the top of the Carnegie ridge (an aseismic ridge that forms the southern boundary of the Panama Basin) may be the source of extra 230Th [2]. Previous geophysical studies also suggest that sediment focusing is a common phenomenon in the Panama Basin [3]. Here, we test the sediment redistribution hypothesis by measuring 230Th in cores located near the tops of the Carnegie ridge and other ridges that bound the Panama Basin (e.g., Cocos). If ridge tops are the source of sediment to the basin, then we expect the 230Th systematics to indicate focusing factors that are less than 1 there. An analysis of the mass balance of 230Th within the Panama Basin will be presented. [1] Lyle et al. 2002, Paleoceanography 17(2), 1013. [2] Kienast et al. 2007, Paleoceanography 22, 2213[3] Lonsdale and Malfait, 1974, Geol. Soc. Am. Bull., 85, 1697-1712
PP23C-1489
Estimates of Late Quaternary Sea-Surface Water Temperature Fluctuations in the Eastern Equatorial Pacific Ocean From an Artificial Neural Network (ANN) Analysis of Radiolarian Data
The radiolarians and other microfossils have been widely used to estimate sea-surface temperatures (SSTs)
by the Imbrie-Kipp Transfer Function (IKTF) method, which evolved in 1971. Besides IKTF, the modern
analog technique (MAT), artificial neural network (ANN), weighted-averaging partial least squares (WAPLS)
regression, and maximum likelihood (ML) methods were used to estimate SSTs based on radiolarian fauna,
and the accuracies associated with these methods were compared. Study suggested that the ANNs represent
more optimum procedure for estimating SSTs than these other methods. Relative abundance (%) data for
38 modern radiolarian species in the core tops from the Pacific Ocean (Pisias et al., 1997) were first
calibrated to measured February-April and August-October SSTs (World Ocean Data CD, 2001) by the ANNs
and the other methods. Average root mean square error of prediction (RMSEP) for the ANNs is considerably
lower (1.5° for February-April and 1.4° for August-October) in 10 independent hold-back (HB)
test sets of observations, that were not part of the training of ANNs, than for the other methods (RMSEP
~2°) using the same split of training and test sets. Correlation coefficients between observed and
estimated SSTs by the ANNs in 10-HB sets for both the February-April and August-October seasons are
0.98. Results suggest that the ANN-derived SSTs are ~25% and ~19% more accurate than those
derived by the other methods during February-April and August-October seasons, respectively. Therefore,
we estimated paleo-SSTs by the ANNs using same radiolarian species down core data in 10 sediment cores
from the eastern equatorial Pacific (Pisias and Mix, 1997), and paleo-SST time series of the last
~800,000 years (~800-ka) were prepared. Besides, the ANN derived SST maps were made for the
modern time, Last Glacial Maximum (LGM, ~23-ka), and Eemian (~125-ka) time-slices in the cores
and analyzed. The ANN based SSTs suggested that the Peru upwelling was stronger during the LGM than at
the modern and the Eemian times.
http://www.nio.org
PP23C-1490
Centennial Scale Variations of Sea Surface Temperature and Productivity Over the last 14 ka in Core PC-14 From the Continetal Margin of Baja California
As an important component of the North Pacific gyre, the evolution of the California Current has important influences on global climate and carbon cycle. We have carried out organic geochemical analyses of core PC-14 off Baja California of the low latitude northeast Pacific to reconstruct centennial scale changes of sea surface temperature (SST) and productivity over the last 14 ka. Both the B/A (Bolling-Allerod) and YD (Younger Drays) events were clearly revealed by the SST record, but the magnitudes of the SST changes were smaller than those recorded in cores from the mid-latitude NE Pacific. The PC-14 SST record for the Holocene did not reveal any obvious trend, in contrast to the decreasing trend from the mid-latitude California margins. The results indicate the low latitude NE Pacific responded to high-latitude climate forcing at millennial scale, but upwelling and ENSO affected low and mid-latitude NE Pacific differently. From the deglaciation to the Holocene, PC-14 productivity shows an overall increasing trend, reflecting a climate transition from an El Nino-dominated climate to a La Nina-dominated climate and increased upwelling. Productivity decreased from the mid- to the late Holocene (6.5-3.8 ka), probably caused by increased ENSO intensities. During the B/A warm period, productivity increased only slightly but the oxygen minimum zone (OMZ) intensified significantly, suggesting that OMZ variations for this region were not controlled by productivity changes.
PP23C-1491
Paleoceanographic Study of the Late Holocene in Carmen and La Paz Basins, Gulf of California
As part of the Gulf of California Paleoceanography Program we have collected sedimentary cores from the basins in the southern Gulf. Here we present results for two anoxic basins and discuss their implications for the Late Holocene paleoceanography. Laminated sediments from two cores from the western slope of Carmen Basin and La Paz Basin provide records of surface and subsurface ocean and climate variability on centennial time scales during the past 1800 yr. The sequences are varved throughout their entire length. Age models, (based on 210Pb chronologies), indicate sedimentation rates of 1.1 mm/yr and 0.07 mm/yr, respectively. Radiolarian, foraminifera, silicoflagellates and magnetic susceptibility are used as proxies of oceanographic and climatic variability. In La Paz Basin radiolarian assemblages suggest a climatic scenario drier and warmer than today between 120 to about 800 AD. The taxa support an oceanographic scenario where water, likely the Gulf of California Water, with relatively high salinity, predominated because of the intensification of evaporation processes. Radiolarians and silicoflagellates reflect ocean surface cooling conditions, most probable, upwelling processes between about 820- 1150 and 1200-1490 AD, as a result of changes in circulation, leading to increased upwelling and enhanced productivity, reflected in a decrease in bottom water oxygenation as indicated by benthic foraminifera. Proxies, in both basins, indicate a warm scenario and the dominance of the Equatorial Surface Water in the southern Gulf suggesting intensification of ENSO cycles, since 1500 A.D. This is supported with the proposal of general increases in SST of 1 to 2°C over the last 300 years, in association with the end of the Little Ice Age. Magnetic susceptibility suggests an increase of terrigenous supply in both basins during the last century. These conditions correlate with ENSO cycles, which have been more intense in the Late Holocene.
PP23C-1492
Ecological Succesion of Planktonic Foraminifera During Upwelling and Non-upwelling Conditions in the Gulf of Tehuantepec
Atmospheric and oceanographic relations are exemplified in the wind - driven upwelling processes that characterize the ocean dynamics of the Gulf of Tehuantepec,Mexican Pacific. To better understand the influence of these processes on planktic foraminifera (a widely used paleoceanographic proxy), we studied their assemblages collected in a sediment trap, located in the path of the wind axis. Twenty samples were collected during the period from late winter (February 2nd) to early summer (July 8th) of 2006. Three species (Globigerina bulloides, Globigerinita glutinata and Globorotalia menardii) constitute over 80 percent of the total association (22 species). From late winter (February 2nd) to middle spring (third week of May) G. bulloides with abundances up to 80 percent, dominates the assemblage. From May 20th, G. menardii, Globigerinella aequilateralis and Globigerinoides ruber increase, and by the end of June G. menardii constitutes nearly 70 percent of the whole assemblage. G. bulloides has been reported as characteristic of upwelling - influenced environments, while G. menardii is related to warm and stratified waters. The dominance periods of these species coincide with satellite images indicating upwelling and warm stratified waters of ecuatorial origin, respectivelly. Biological productivity is also correlated with highest numbers of up to 2200 individuals/l/m2 during the upwelling season, and higher concentrations of chlorophyll-a.
PP23C-1493
Towards the Reconstruction of the Thermocline Ridge in the Western Tropical Pacific: Implications for LGM Tropical Atmospheric Circulation
Several studies have suggested that the intertropical convergence zone (ITCZ) shifts south in response to cooling the northern hemisphere during the Last Glacial Maximum (LGM). However, there have been few attempts to constrain the position of the ITCZ during the LGM with paleoclimate data. Since zonal wind patterns across the Pacific largely drive the approximately geostrophic currents near the equator, we would expect that a shift in the ITCZ would cause a shift in current positions, and therefore a shift in the seawater density gradients associated with the currents. The oxygen isotope ratio in planktonic foraminiferal calcite is sensitive mainly to the temperature and the oxygen isotope ratio of the seawater in which it was formed. Specifically, the species Globorotalia tumida calcifies at approximately 150 m depth, which makes it ideal for reconstructing the lateral density gradients in the upper ocean associated with the wind driven currents. We determine the oxygen isotope ratios in G. tumida for the recent and glacial sections of a series of sediment cores which transect the equator at approximately 145-150°E. We use this data to locate the thermocline ridge which characterizes the boundary between the Equatorial Countercurrent and the North Equatorial Current. Our transect currently runs from approximately 4°S to 11°N. The core top data appear to resolve the ridge well, matching expected oxygen isotope values calculated from instrumental salinity and temperature data. So far, we have identified the LGM based on planktonic foraminiferal isotope stratigraphies. The LGM data appear to show that the thermocline ridge is not significantly displaced southward relative to today, although our transect does not yet continue far enough north to show that the location of the LGM ridge is identical to the modern.
PP23C-1494
Late Quaternary Paleoceanography, Productivity, and Sedimentation in the Western Tropical Pacific, Gulf of Papua
The Gulf of Papua (GoP) is a tropical mixed carbonate-siliciclastic system located at the northern limit of the Great Barrier Reef and the southern edge of Papua New Guinea where large volumes of terrigenous material are deposited in a region characterized by abundant neritic, pelagic, and benthic marine carbonate production. GoP sediments contain an important record of late Quaternary climate and oceanographic changes at the edge of the Western Pacific Warm Pool, in an area seasonally influenced by movement of the Intertropical Convergence Zone and the effects of the El Niño Southern Oscillation. The GoP is also the crossroads for a number of major water masses (e.g., Antarctic Intermediate Water, South Equatorial Current, New Guinea Coastal Undercurrent) that potentially play a role in glacial-interglacial and millennial scale climate variability as well as the transfer of high latitude climate signals to the low latitudes. A multiproxy approach is used to describe how changes in oceanography and climate influenced sedimentation in the GoP over the last several glacial-interglacial cycles. Carbon and oxygen isotope records from planktic (G. ruber) and benthic (C. wuellerstorfi) foraminifera are used to assess changes in nutrient concentrations, productivity, water column structure, and climate influences on the surface and deep waters. Elemental records measured with an X-Ray Fluorescence (XRF) core scanner show high variability in both pelagic and neritic carbonate production (Ca/Fe and Sr/Ca) and productivity (e.g., Ba/Ti) between and within glacial- interglacial periods. High carbonate content and variability generally occur during the interglacial periods. However, MIS 6 also has relatively high levels of inferred carbonate production and productivity in contrast to low levels during MIS 2. Variations in elemental intensity ratios (e.g., Fe/Ti and Mn/Ti) can be used to infer changes in sediment source and/or the oxygen content of the water column and sediments. Late Quaternary sedimentation, productivity, and oceanography of the GoP reflect complex interactions between changes in sea level, local climate, and global climate change.
PP23C-1495
Changes in vertical hydrological profile at the southern margin of the Western Pacific Warm Pool (WPWP) during the past 168,000 years
Here we present stable isotope and Mg/Ca elemental ratio records of planktonic foraminifera G. sac and P. obli from the MD05-2925 (9°20'S, 151°27É , water depth 1660 m) of the Woodlark Basin on the southern margin of the western Pacific warm pool (WPWP) for the past 168,000 years. The oxygen isotope ratios of G. sac and P. obli fluctuated with an amplitude of 1.0 -1.5 ‰ during the past two glacial-interglacial (G/IG) cycles. The G. sac δ13C record shows a significant precession cyclicity of 21-kyr years, whereas P. obli δ13C shows the periodicity of 41-kyr cycle. The Mg/Ca ratios of G. sac and P. obli fluctuate with a range of a ~3 and ~5°C, respectively. The resultant sea-water δ18Ow values derived from both species show good agreement in amplitude and absolute value. Spectral analyses of δ 18Ow recoords show periodicity of 21-kyr cycle. The results of spectral analyses of δ13C and Mg/Ca-derived upper thermocline temperature (UTT) suggest a strong influence of high latitudes climate on the sub-surface hydrographic changes at this site. The decoupling of surface and sub-surface water in temperature during the early IG periods can be explained by the dramatically thickening of mixed layer and deepening of the thermocline during the second half of the Terminations, assuming that the dwelling depth of P. obli did not change much. The good similarity in δ18Ow between surface and sub-surface species may imply that the stratification at this site is mainly caused by temperature gradient rather than salinity. However, the relationship between the southern high-latitude climate and the sub-surface waters in the WPWP region during the G/IG needs further examination.
PP23C-1496
A 130 Thousand-Years Record of the East Asian Summer Monsoon from the South China Sea
Piston Core MD972151 (9°N) obtained from the South China Sea off the southern end of Indochina Peninsula has offered a multiform of high-resolution records including planktic and benthic foraminifera δ18O, alkenone-based SST, and %CaCO3. This study constructed an independent chronology differing from the conventional SPECMAP time-scale and compared the various proxy records to the East Asian monsoon proxy of Chinese speleothem (Wang et al., 2008) over the past 130 kyrs. Among the various proxies evaluated, the profile of seawater 18O displays remarkable similarity with the Sanbao-Hulu speleothem record. The 25 Chinese interstadial events (CIS) events (A1-A25) during the last glacial period as well as the various cold stadial intervals, such as the Younger-Dryas and five Heinrich events (H1-H5.2) are readily recognizable. After removing the long-term ice-volume related variation, the residual sea-water δ18O shows a strong precession period of 23 kyrs, indicative of dominant control of low-latitude solar insolation. We interpret that the southern South China Sea serves as a major vapor source for the East Asian Summer Monsoon, and the positive feedback through the increased runoff from the Indochina Peninsula may further amplify the summer monsoon signal in the southern South China Sea.
PP23C-1497
Sea Surface Salinity Record of the Last 260 ka from the Northern South China Sea
MD05-2904 from the northern South China Sea (SCS) provided detailed U k' 37 Sea Surface Temperature (SST) and planktonic foraminifer G. ruber Δ 18 O records during the past 260 ka. In general, the Δ 18 O G. ruber record parallels the SST record on major glacial/interglacial cycles. Notable differences between the two records were in MIS 7.4, 6.5, 5.5 and early part of 3, when the Δ 18 O G. ruber record 'deviated' from the SPECMAP or LR04 curves (Martinson et al., 1987; Lisiecki and Raymo, 2005). Δ 18 O residue, the oxygen isotope residual and a paleosanlinity proxy, was calculated and used to illustrate the differences. Generally paralleling the Δ 18 O G. ruber curve, the Δ 18 O residue record shows relatively high values during MIS 5.5, and low values during MIS 7.4, 6.5 and early part of MIS 3. While the Δ 18 O residue record shows obscure glaciations / glaciation patterns, spectrum analysis reveals its strong precession (23.8 kyr) and semi-precession (9 kyr) cycle. By comparison with the stalagmite Δ 18 O and the global relative sea level records, the Δ 18 O G. ruber 'deviations' are attributed to sea surface salinity variations which are related to East Asian Summer Monsoon (EASM) changes. Thus during MIS 7.4, 6.5 and early part of MIS 3, strong EASM lead to increased precipitation and resulted in relatively lower Δ 18 O residue; while during MIS 5.5, comparatively weak EASM and combined with higher evaporation, caused high Δ 18 O residue. Its significant precession signal and its close correlation with the EASM index suggests that the Δ 18 O residue record is a good proxy for EASM in the northern South China Sea.
PP23C-1498
Changes in Phytoplankton Community Structure in Western South China Sea Over the Past 450 Kyrs
Molecular biomarkers (brassicasterol, dinosterol, alkenones, C30 1,15-diol/keto-ol and cholesterol) were measured in an IMAGES core MD05-2901 (14°22.50'N, 110°44.60'E, w.d. 1454 m), to reconstruct the phytoplankton community structure over the past 450 kyrs in the western South China Sea on the eastern slope off Vietnam. Our results showed that diatoms, which are most sensitive to nutrition supply, exhibited relatively high productivity in interglacial period and lower productivity in glacial period, possibly caused by summer monsoon-induced upwelling. Dinoflagellates exhibited simillar phenomena as diatoms. But, coccolithphorids did not show clear glacial/intergalcial patterns and had a gradual increasing trend from 450 kyrs to 200 kyrs followed by a gradual decreasing trend until the present. Eustigmatophytes exhibited higher contents in glacial than interglacial periods and almost the same variations with terrestrial n- alkanes. Relationships among the major biomarker groups indicated complex responses of different phytoplanktons to changes in paleoclimate and paleoenvironment in the past 450 kyrs in western South China Sea.
PP23C-1499
The Latitudinal and Vertical Thermal Distribution Change from the Last Glacial Maximum in the Western North Pacific
We conducted multi-species analysis of planktonic foraminiferal oxygen isotope and Mg/Ca in the tropical and subtropical western North Pacific sediment core in order to investigate latitudinal and vertical thermal structure change from the LGM. A box core 3cBX was collected from the west Caroline Basin (8 01 N, 139 38 E), and a piston core ASM5 was collected from the Amami Sea Mount (28 23 N, 132 45 E). Eight and seven species of planktonic foraminifera were picked from 3cBX and ASM5, respectively, in order to analyze oxygen isotope. In the tropics, the glacial-interglacial amplitude of G. ruber oxygen isotope was approximately 1.0 per mil between LGM and Holocene. On the other hand, the amplitude in the subtropics was approximately 1.5per mil. Because G. ruber prefers summer warmest temperature, the oxygen isotope difference suggested that the latitudinal summer surface temperature/salinity gradient in the last glacial period was steeper than that of modern. Multi-species approach reveals that the vertical thermal structure variations in the North Pacific. The vertical thermal gradient in the subtropic region was gentler in the LGM and steeper in the last deglaciation than modern condition. We will discuss about Mg/Ca temperature and salinity variation in the presentation.
PP23C-1500
South American Pluvial Lakes: Implications for Quaternary Climate Change
Pluvial lakes in Argentina provide a unique record of climate change during the Late Quaternary. Two lake systems were studied as part of an effort to understand major climatic changes during the Last Glacial Maximum (LGM: 24-18 ka) and the following deglaciation (18-13 ka). Today, both of the studied regions are arid, and the lake basins contain much smaller lakes than in the past. Laguna Cari-Laufquen is located at 41°S and the modern basin is occupied by two lakes: Cari-Laufquen Grande and Chica. Preserved shorelines and sediments show these two lakes rose and merged multiple times during the Late Quaternary. Initial radiocarbon and U-series dates show the lake was the highest during the LGM, with several smaller, more recent lake oscillations. The second lake system in this study is Laguna Bebedero, located at 33°S in western Argentina. Previous studies show the highest shorelines date to 23-16,000 yrs BP. Our new radiocarbon dates help constrain the timing of the highstands, allowing the differentiation between the LGM-age lake, and lakes created during the following deglaciation. Our lake highstand evidence provides a unique opportunity to reconstruct shifts in Atlantic (easterly)-or Pacific (westerly)-driven moisture. The age of deep lakes at Laguna Cari-Laufquen and Bebedero suggest a more northerly position of the westerlies during the LGM. The late-glacial age of the high shorelines at Bebedero points to a shift in the easterlies to a more southerly position. The late-glacial intensification and expansion of the sub-tropical easterlies is visible in many other South American lake records, and is probably linked to a colder eastern Pacific and North Atlantic at that time.
PP23C-1501
Theoretical model for the control of Pleistocene moisture availability in the Tropics: combining independent movement of north and south boundaries of the ITCZ and precessional forcing.
Paleoclimate records of tropical moisture availability suggest there are complex controls. Using marine and continental records from both South America and Africa it is possible to resolve these influences and start to build a theoretical model explaining variations in both rainfall and moisture availability. The first control is the position of the Intertropical Convergence Zone (ITCZ). Evidence is emerging that the northern and southern boundaries of the ITCZ move independently. The extreme seasonal position of the boundary is controlled by the temperature gradient between the Equator and the relevant Pole. The temperature gradient of each Hemisphere is governed primarily by the prevalent boundary condition i.e., whether it is a glacial or interglacial period. The secondary influence are millennial-scale changes such as the Heinrich events. This idea is important as it moves away from the concept that the ITCZ is a fixed band which moves north and south. The second major control is precession which influences seasonality in the Tropics. This is important as it controls the strength of convection in each Hemisphere and thus the strength of the resultant monsoon. For example Amazonia monsoon is controlled by Southern Hemisphere convection strength, while the Congo and SE Asia monsoons are controlled by the Northern Hemisphere. In terms of tropical rainfall it has been shown by GCMs that precession can have the same scale of affect as switching from a glacial to an interglacial period. In summary the relative position of the northern and southern boundaries of the ITCZ controls the location of rainfall. While precession controls the intensity of the convection within the ITZC and thus the strength of the monsoon. This radical new theoretical framework explains why rainfall and moisture records from the same region e.g., Amazonia can be very different on a millennial and centennial time-scale. New evidence from Amazonia and East Africa combined with ice core data will be presented to support this new theoretical model.
PP23C-1502
Teasing Apart Regional Climate and Meltwater Influences on Florida Straits Sea Surface Temperature and Salinity over the past 40 kyr
Recent reconstructions of North Atlantic salinity variability over the last glacial cycle show that abrupt climate events are linked to major reorganizations in the low-latitude hydrologic cycle, affecting large-scale changes in evaporation minus precipitation (E-P) patterns. Although there is general agreement that the Intertropical Convergence Zone (ITCZ) migrates southward during cold stadials, it remains unclear how this shift affects the net E-P budget in the North Atlantic. In order to reconstruct a high resolution record of past sea surface temperature (SST) and salinity (SSS) in the Florida Straits across abrupt climate events of the last 40 kyr, we combine Mg/Ca paleothermometry and δ18O measurements in shells from the surface-dwelling foraminifera Globigerinoides ruber in cores KNR166-2-JPC29 (24°17'N, 83°16'W; 648 m depth; 8-20 cm/kyr sed. rate) and JPC26 (24°19.61'N, 83°15.14'W; 546 m depth; 18-240 cm/kyr sed. rate) and calculate δ18OSEAWATER (δ18OSW) variability. Removal of the δ18OSW signal due to continental ice volume variation results in the ice volume-free (IVF) δ18OSW record (a proxy for SSS variability). Although most waters flowing through the Florida Straits today originate in the tropical western Atlantic, major meltwater discharges from the Mississippi River across the last deglacial period also influenced SST and SSS in the Florida Straits. To constrain periods of increased meltwater discharge, we measured Ba/Ca ratios in G. ruber from select intervals. Because riverine waters have a much higher dissolved Ba+2 concentration relative to seawater, foraminifera Ba/Ca ratios can be used as an additional proxy to constrain periods of increase riverine discharge. Initial results suggest the hydrographic history of the Florida Straits is influenced by both meltwater discharge and regional climate variability linked to the high-latitude North Atlantic. Both the IVF- δ18OSW and Ba/Ca records reveal a prolonged period from 16.0-13.0 kyr when elevated meltwater discharge was the dominant influence on surface water conditions in the Florida Straits. It is likely that SSS in the Florida Straits was significantly fresher than today during this interval. In contrast, periods of minimal meltwater influence (such as the Younger Dryas and across D-O cycles of MIS 3) are characterized by abrupt SST and SSS shifts that covary with the NGRIP δ18Oice record. SSTs in the Florida Straits cool by 1.5-2.0 °C and regional salinity increases (IVF-δ18OSW increase of 0.5-0.7‰) at the initiation of cold stadial events as the ITCZ shifts south. The most likely explanation for these rapid shifts in IVF-δ18OSW values is that moisture transport out of the North Atlantic increases when the North Atlantic cools and the ITCZ shifts southward.
PP23C-1503
Variations of chemical components in fossil corals collected from Tahiti by IODP Expedition 310
Integrated Ocean Drilling Program (IODP) Expedition 310 to the reef terraces around Tahiti, French Polynesia was conducted for the purpose of establishing the course of postglacial sea level rise at Tahiti and to define sea-surface temperature (SST) and marine environmental variation for the region over the period 20-10 ka. While sea-level during the Last Glacial Maximum (LGM) has been observed as much as ca. 130 m below its present value for sites remote from former ice sheet (far-field sites), fossil coral records contain evidence of a dramatic sea level rise in excess 20 m within the last deglaciation around 14 ka, termed meltwater pulse 1A (mwp-1A). Although this event would have had a significant effect on global climate, the relationship between mwp-1A and the climate system remains a subject of debate. Coral geochemistry has a potential as proxies for marine environments, and in this study we measured trace elements, such as Ba, U and Cd, in addition to Pb isotope ratios in fossil Porites sp. collected around Tahiti in order to reconstruct SST, upwelling and/or wind source/direction. Skeletal conditions of fossil corals were investigated using X-ray diffraction and we selected samples composed of only aragonite. Dating of fossil corals were determined based on 14C method by accelerator mass spectrometry. As a result, approximately 10 samples were from before and in the period of mwp-1A event (15 - 14 ka). Then bulk samples were taken from all coral specimens by a milling machine and trace elements and Pb isotopic compositions in bulk samples were analyzed by inductively coupled mass spectrometer (ICP-MS) and multiple-collector ICP-MS, respectively. Data of Pb isotope ratios in fossil corals were comparable with those from crust in the Central Andes, suggesting that easterly wind was predominant around Tahiti with a slight variation depending on the age. We will also discuss the relationship between upwelling and climate condition around the mwp-1A based on Cd contents and U/Ca ratios.
PP23C-1504
High-Resolution Stable Nitrogen and Carbon Isotope, and Trace Element Records From a Western Tropical Pacific Near-Surface Black Coral and a Mid-Water Soft Coral
In contrast to the well-studied eastern tropical Pacific Ocean, details of the effects of the El Nino Southern Oscillation (ENSO) on mid-water oceanography in the western tropical Pacific are less well understood and would be enhanced by high resolution, century-scale proxy records. Soft corals and black corals are found in the western tropical Pacific from surface waters to thousands of meters deep, deposit organic skeleton in concentric bands, and live for hundreds to thousands of years. The geochemical composition of their organic skeleton is driven primarily by the geochemical composition of organic matter in the water column upon which they feed. Therefore, soft corals and black corals can be used to reconstruct paleoceanographic records of the geochemistry of organic matter across depths. Here, we measured the stable isotopic (δ15N and δ13C) and trace element composition in the organic skeleton of a soft coral (5 m depth) and a black coral (85 m depth) collected 2 km offshore from Palau in the western tropical Pacific. Measurements were made at sub-annual resolution across a radial growth axis at the base of each coral colony. Using radiometric dating, a growth chronology was developed for the two colonies, and applied to skeletal stable isotope and trace element time series. The δ15N record from 85 m showed a decrease of 1 permil over the 100-year record. By comparison, the δ15N record from 5 m was characterized by a 1 permil increase and subsequent decrease over the 20-year record. δ13C record from both depths displayed a decrease in δ13C values comparable to that measured in atmospheric CO2 δ13C (i.e., Suess effect). Trace element analyses of these samples are in progress and will be presented. Implications of the δ15N, δ13C and trace element records will be discussed relative to temperature datasets, ENSO indices, and thermocline variability in this region. This research is the first to use both stable isotope and trace element measurements from soft corals and black corals, and to produce a high resolution paleo-reconstruction of organic matter geochemistry from mid-water depths.
PP23C-1505
Evaluating the Effect of Coral Topography on the Climate Signal in Porites spp.
Understanding how the record of environmental variations in the surface waters of tropical oceans is encoded in the skeletal geochemistry of corals is essential to deciding how to extract a signal from their skeletons. A few previous investigators have studied coral growth patterns and corresponding geochemical signals; however despite this attention, questions remain about the best way to extract a climate record from coral cores with regard to skeletal topography and drill path location. Utilizing a slabbed core of Porites lutea from Espiritu Santo, Vanuatu (~15°06'S, 166°52'E) we extracted coral samples along growth bands to evaluate geochemical variations within skeletal isochrons. Coral samples were analyzed using an inductively coupled plasma-optical emission spectrometer to determine Sr/Ca ratios, a proxy for sea surface temperature (SST.) Ninety-three Sr/Ca determinations were made from the first isochron; the average value was 8.982 ± 0.030 mmol/mol. The variation about the mean is nearly identical to the analytical precision associated with the Sr/Ca determination. These results suggest that drilling a diachronous path along any axis of growth in a coral skeleton should yield similar results. A second isochron extracted from the coral is being processed to replicate these initial results. In a second experiment, utilizing another slabbed core of Porites lutea from Efate, Vanuatu (~17°34'S, 168°14'E) two sets of paths were drilled through sections of coral characterized by pronounced skeletal topography. In each of these pairs, one path was drilled along an axis of minimum growth and another along an axis of maximum growth. Coral samples were analyzed in the same way as described above to determine Sr/Ca ratios and these analytical determinations are being compared to the instrumental record of SST variations in the region. These results will show whether growth topography is an important variable to monitor when extracting a climate signal from Porites lutea.
PP23C-1506
Tropical Climate Variability and the Atlantic Multi-decadal Oscillation from the Geochemistry of Corals and Sclerosponges over the last 500 years
Observational and model data have shown considerable variability to exist in North Atlantic sea surface temperatures and climate. In particular, several climate oscillations have been documented, including the North Atlantic Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO). While the exact forcing of these modes has yet to be defined, they do appear to produce observable climatic variability on multi- decadal time scales. Recent geochemical analyses on climate proxy archives from the western tropical Atlantic have shown variability consistent with that of the AMO. A 300+ year old coral specimen of Montastraea faveolata growing in 6m of water south of Port Everglades, Florida, and a 550+ year old sclerosponge specimen of Ceratoporella nicholsoni collected from 133m of water off of Lee Stocking Island in the Exuma Sound, Bahamas, have been milled at a sub- annual resolution and analyzed for stable C and O isotopes as well as minor element ratios. Salinity calculated from a combination of δ18O and Sr/Ca exhibits similar periodicities to the AMO between ~1700 AD and the present; a rather surprising finding given the significant differences in the local environmental conditions of the two samples. Prior to the 1700s, our sclerosponge record diverges from the AMO record reconstructed from tree rings. Additional analyses from the region will be needed to determine the reasoning for this discrepancy.
PP23C-1507
Sea Surface Temperature from the Western Pacific Warm Pool (Misima Island, Papua New Guinea) using the Geochemistry of Modern and Pre-industrial Corals
The Western Pacific Warm Pool (WPWP) is a major driver of the global climate system because it collects, harbors, and transports vast amounts of energy in its warm water to the extratropics by atmospheric and oceanic circulation. Despite the climatological significance of the WPWP, sea surface temperature (SST) records from this region are sparse. This study begins to address this data gap by developing proxy SST records using the geochemistry of modern and pre-industrial corals from Misima Island, which is located ~200 km east of the mainland of Papua New Guinea. The first goal of this project is to establish that robust proxy climate records can be generated from a modern Porites coral from Misima. Initial work is focused on generating monthly resolved Sr/Ca records from modern Porites coral samples. Coral Sr/Ca data generated to date document the presence of inter- and intrannual variability; stable isotopic determinations are in progress. These records will provide the modern proof of concept, which will be used in the second phase of this project that seeks to reconstruct climate variability in the WPWP from a suite of pre-industrial corals (~500 to 1000 year old) from Misima.
PP23C-1508
Stalagmite δ18O Variability Records of ENSO-Controlled Rainfall From Niue Island, South Pacific
Niue Island (19°00'S, 169°50'W), a large carbonate platform located at the edge of Pacific Warm Water Pool offers an ideal opportunity to reconstruct El Nino Southern Oscillation (ENSO) history by means of speleothems. Rainfall on Niue is primarily controlled by ENSO. Previously employed layer thickness proxy revealed that ENSO phenomenon is archived in stalagmites from Niue (Rasbury et al., 2006). Here we present a high resolution (sub-annual) δ18O profile from a stalagmite (ASM1) sampled in 2002 in a active growing position from a flank margin cave on Niue Island in order to test whether oxygen isotope composition of Niue stalagmites is a reliable proxy tool for detecting rainfall and ENSO variability. The stalagmite contains approximately 150 years of deposition and consists of sub-annual couplets alternating between thick, light calcite bands deposited during the austral summer and thin, dark calcite layers deposited during the austral dry winter. Comparison of ASM1 stalagmite δ18O with Niue annual rainfall generally shows an inverse relationship between oxygen isotope ratios and the amount of precipitation, with 18O depleted values reflecting higher rainfall. Conversely, 18O enriched values usually correspond with low rainfall. Because ENSO results in severe droughts on the island, Southern Oscillation Index (SOI) was also evaluated against ASM1 stalagmite δ18O. Not surprisingly, low SOI phases (ENSO events) coincide with more positive δ18O values. Thus, stalagmite δ18O of ASM1 represents a proxy tool for the amount of rainfall on Niue and ENSO variability. Ongoing research aims to derive an additional δ18 high resolution profile from a coeval stalagmite. Power spectra analysis of the δ18O profiles will be presented at the meeting. Reference: Rasbury, M., and P. Aharon (2006), ENSO-controlled rainfall variability records archived in tropical stalagmites from the mid-ocean island of Niue, South Pacific, Geochem. Geophys. Geosyst., 7, Q07010, doi:10.1029/2005GC001232.
PP23C-1509
High Resolution Coral Records of Climate Variability in the Southwest Pacific Ocean During the Last Glacial Maximum and the Younger Dryas.
Corals constitute the ideal complement to traditional deep-sea core paleoclimatic records based on planktonic organisms because they offer the possibility to obtain high resolution records. This characteristic of corals is especially important to study the variability of parameters such as Sea Surface Temperature (SST) or the hydrological cycle on annual to decadal timescales. However, most coral studies focussed on the past millennium, because corals are prone to diagenesis, and it is difficult to find pristine corals prior to the Holocene. Here, we present two coral records from Vanuatu, in the Southwest Pacific Ocean, dated from the Last Glacial Maximum (21970±76 yr BP) and the onset of the Younger Dryas (12610±49 yr BP) periods. The amount of cooling in the tropics during these two cold periods is still a matter of debate. We performed coupled stable isotopes (δ18O) and trace elements (Sr/Ca and U/Ca) analyses as well as a novel approach using digital X-raying to reconstruct past oceanic parameters. Our records suggest that the LGM was 2 to 2.5° C and the beginning of the Younger Dryas about 1° C colder than today. Coupled to these lower SST, we document a difference in the hydrological cycle. Reconstructed seawater (δ18O) values seem to indicate a global reduction in precipitation, and an annual minimum linked to the highest SST. This situation is opposite to the one experienced today in Vanuatu, where the South Pacific Convergence Zone (SPCZ) is more active during the warmest months. The LGM and YD situation resembles that of El Niño episodes, when the SPCZ migrates to the north and merges with the Inter Tropical Convergence Zone (ITCZ), lowering precipitations over Vanuatu.
PP23C-1510
Comparison of a 1500 Year-Long ENSO-Sensitive Ice Core Proxy Record to Modeled Solar Forcing Mechanisms
A calibrated 1500 year-long glaciochemical proxy record for the strength of the wintertime (November-March) Aleutian Low (ALOW) from the Mt. Logan summit (PR Col; 5300 m a.s.l.) ice core [Na+] time series reveals a strong ALOW from ca. 650-900 A.D., ca. 1300-1550 A.D., and ca. 1700-1998 A.D., and a weaker ALOW from ca. 900-1300 A.D. and ca. 1550-1700 A.D. The proxy record was calibrated to instrumental sea-level pressure data using standard regression techniques, and verified using statistical, spectral, and spatial correlation analyses. This record is consistent with ENSO-sensitive paleoclimate proxies from the Pacific basin that indicate El Niño-like conditions during the Little Ice Age (LIA) and La Niña- like conditions during the so-called Medieval Climate Anomaly (MCA). The Mt. Logan ALOW proxy record is significantly correlated with atmospheric Δ14C records (r = 0.39, p<0.05), supporting an influence of solar forcing on late Holocene North Pacific climate. The increased frequency of El Niño conditions during the LIA relative to the MCA is consistent with the modeled "ocean thermostat" mechanism whereby weak solar irradiance during the LIA promotes El Niño-like conditions, and strong MCA solar irradiance promotes La Niña-like conditions by modifying the east-west SST gradient in the tropical Pacific. We will also discuss the consistency of these paleoproxy records with models invoking solar-induced changes in the north-south temperature gradient at the tropopause. In these model results, weaker solar irradiance causes cooling and weaker westerly winds in the stratosphere, inducing a tropospheric response that includes expansion of the polar cell, contraction of the Hadley cell, and enhanced meridional circulation.
PP23C-1511
Eclipse Ice Core Accumulation and Stable Isotope Variability as an Indicator of North Pacific Climate
Three ice cores retrieved from the Eclipse Icefield in Yukon Territory, Canada provide a unique, high- resolution record reflecting climatic and environmental conditions in the North Pacific and western-North American region. A stacked core record was produced to filter noise and extract the dominant weather and climate signals. Accumulation and stable isotopes records were studied and calibrated with modern instrumental data from 1948-2001 to determine their level of utility in reconstructing past precipitation, 500 hPa geopotential heights and surface temperature patterns. Results reveal that the most extreme years and seasons of precipitation are strongly influenced by the average amplitude and position of the Rossby wave in the eastern Pacific region and western-North America. High (low) accumulation is associated with a stronger (weaker) mean Aleutian low and stronger (weaker) mean western-North American ridge. The sign of the PNA correlated well during these extreme accumulation periods, as did the Multivariate ENSO Index to a slightly lesser degree. This result strongly supports extreme accumulation years as a proxy for North Pacific basin atmospheric circulation. Composites of 500 hPa geopotential height anomaly patterns during high and low years of stable isotope fractionation yield high interannual variability, thus calling into question the climatic reconstruction utility of stable isotopes from this drillsite.
PP23C-1512
Low-Frequency Tropical Pacific Sea-Surface Temperature over the Past Millennium: Reconstruction and Error Estimates
Tropical Pacific sea-surface temperatures can organize climate variability at near-global scales, and since there is wide disagreement over their projected course under greenhouse forcing, it is of considerable interest to understand their evolution over the past millennium. We use the most recent high-resolution proxy data from ENSO-sensitive regions, together with the RegEM climate field reconstruction technique [Schneider, 2001, Rutherford et al, 2003, Mann et al, 2007], to extend the history of the NINO3 index at decadal scales through 1000 A.D. We present a new algorithm implementing an objective regularization technique that preserves low-frequency variance in RegEM (ITTLS).Synthetic SST and pseudoproxy tests using a realistic ENSO model are used to test the accuracy of estimated low-frequency tropical climate variability with this method The reconstruction shows important decadal and centennial variability throughout the millennium, in the context of which the twentieth century does not appear anomalous. We analyze the sensitivity of the reconstruction to the inclusion of various key proxy timeseries, target SST datasets, and subjective procedural choices, with a particular focus on representing uncertainties. By some measures, the reconstruction is found skillful back to 1500 A.D., but increasing uncertainties in earlier times may limit our ability to test proposed mechanisms of mediaeval climate variability.
PP23C-1513
Low-frequency response of the Arctic Oscillation to external forcing in the past millennium
The Arctic Oscillation (AO) has been analyzed in simulations with the global climate model ECHO-G covering the past millennium and this century, to identify the timescales at which signal of the past external forcing may arise above the internal climate variability. Despite the strong AO-sensitivity of ECHO-G to forcing in the scenario simulations, most of the past multidecadal variability of the model AO is internally generated, and thus at these timescales simulations and proxy-based reconstruction should not necessarily agree. Most recent observed trends in the AO could therefore not be related to external forcing. The influence of past external forcing is probably discernible in the AO at multicentennial timescales. The simulated AO in the initial and final centuries in the millennium is stronger than in the central centuries from about 1400 onwards, in agreement with multicentennial proxy records related to the regional manifestation of the AO, the North Atlantic Oscillation.
PP23C-1514
New Perspective on the Little Ice Age in South America
The d18O of two speleothems from northeastern Peru provides a 1000 year history of precipitation variability at subannual to decadal resolution. The speleothem d18O varies in response to shifting North Atlantic SSTs that bring about a southward or northward bias in the Intertropical Convergence Zone. A southward bias in the ITCZ leads to increased precipitation over eastern Peru as more tropical Atlantic storms impact the region. Wetter periods cause lower speleothem d18O due to the ¡¥amount effect¡¦. The 1000 year speleothem record documents a multi-century increase in the amount of precipitation during the ¡§Little Ice Age¡¨. There has been a progressive decrease in precipitation over the past two centuries. This speleothem shows that precipitation was higher during the Little Ice than at any other time in past 1000 years. Increased precipitation in the LIA coincided with a glacial advance in the Venezuelan Andes. These results require a reinterpretation of the Cariaco Ti. Reduced Ti% in Cariaco during the LIA was previously interpreted to reflect reduced rainfall and runoff over South America. However, the speleothem ƒÔ18O record together with evidence of advancing glaciers imply that during the LIA much of the reduced river runoff may have been the results of increased snow and ice that locked sediments and Ti% in their source regions. As the climate warmed melting glaciers may have released Ti to rivers flowing towards the Cariaco Basin.
PP23C-1515
Late Neogene sea Surface Temperatures in the Sub-tropical North Pacific
The lack of quantitative sea surface temperature (SST) estimates from the marine sedimentary record have made it difficult to understand the causes of relatively warm climate during the late Miocene and Pliocene epochs. To better characterize the SST history of the open ocean North Pacific we used alkenone paleothermometry to determine SST changes at two North Pacific subtropical sites, ODP Site 1208 and ODP Site 1021. Both SST records include continuous late Miocene through early Pliocene sections (~9- 4.5Ma); however, the Site 1208 SST record is continuous through the present. The results are the first SST data from the North Pacific at sites that are not strongly influenced by regional upwelling, and verify a long- term trend of cooling temperatures since ~9.3Ma. However, the magnitude of cooling, at least through the Pliocene, was not as great as that in upwelling regions. During the warm Pliocene (~3.2-5.1Ma) SSTs at ODP 1208 were an average of 21.5°C. These SSTs are only ~1.4°C warmer than the modern temperatures at the site's mid-Pliocene paleolocation. The absence at Site 1208 of a large warming such as that observed at ODP Sites 1014, 1237, 1084, and 846 helps confirm that the substantially warmer SSTs of the Mid-Pliocene are distinct features of upwelling regions. In contrast, the average SST at Site 1208 9.3-7Ma was 23.5°C, which is ~3.5° warmer than the modern SSTs at the site's late Miocene paleolocation. Similarly, late Miocene SSTs from ODP 1021 are >5°C warmer than modern SSTs. The relatively high SSTs throughout the late Miocene North Pacific are notable because estimates of late Miocene atmospheric pCO2 indicate that concentrations were not significantly higher than pre-industrial values. These observations imply that an alternate mechanism must have been responsible for maintaining global warmth during the late Miocene.