PP44A-01 INVITED 16:00h
Deep Ocean Temperature and Salinity at the Last Glacial Maximum
Sediment pore fluids from the deep ocean contain a record of the bottom water salinity and $\delta$$^{18}$O due to the last glaciation. Storage of water on land as glacial ice imparts a global signal of increased salinity and enriched oxygen isotopic values in the abyssal ocean and changes in deep circulation patterns can impart an additional local signal. In the mean this signal was about 3.5% in salinity and about 1.0\permil in $\delta$$^{18}$O, but has been diffusing and advecting away over the last 20,000 years. Today, high resolution sampling (every 1.5 meters) coupled with high precision geochemical analyses of pore fluids from a single core can be used, in conjunction with a 1-D pore fluid diffusion model, to reconstruct the Last Glacial Maximum (LGM) salinity and $\delta$$^{18}$O at that site. We have done this analysis for several globally distributed Ocean Drilling Program (ODP) holes including some new unpublished data from the equatorial Pacific. In addition, benthic foraminiferal measurements of modern and LGM CaCO$_{3}$ $\delta$$^{18}$O, coupled with the water $\delta$$^{18}$O reconstruction, can be used to constrain the deep-water temperature change from LGM to today. Plotted as a T/S diagram for the LGM, our data show that the entire deep ocean cooled to about $-$$1.0\deg$C potential temperature. This relative homogeneity in temperature is contrasted by much larger than modern salinity gradients in the deep. Due to increased sea ice export, the Southern Ocean was by far the saltiest water mass in the LGM, and as a result the modern salinity contrast between NADW and AABW was reversed. Stratification of the LGM deep ocean was largely controlled by salinity (as opposed to temperature today) and this feature has important implications for the stability of the LGM overturning circulation. With salty waters filling the deep LGM Ocean, it is difficult for fresh water changes at the surface of the North Atlantic to alter the abyssal circulation. Before freshwater forcing can trigger circulation changes, some other forcing must first increase the buoyancy of the deep Southern Source waters. In addition, the combination of our water $\delta$$^{18}$O data and a global database of benthic $\delta$$^{18}$O imply that the deep Pacific was warmer than the deep North Atlantic at the LGM.
PP44A-02 INVITED 16:15h
Global Ocean Sensitivity to Local Geologically Short-Term Variability of Freshwater Fluxes
The geologic record and computer modeling indicate that the transitions between cold and warm climates during the last deglaciation , driven by internal climate dynamics, were geologically very fast, lasting for only decades or shorter. The THC is, perhaps, the only viable candidate for driving these kinds of abrupt changes. Current perception of how the THC may become an agent of abrupt climate change is that the THC is rather sensitive to changes in freshwater fluxes in the high-latitudes, also known as major meltwater events. Our recent numerical experiments challenge the idea of the high-latitudinal meltwater events as the only possible cause of THC alteration. These experiments suggest that the inter-basin sea surface salinity contrasts caused by disparity of freshwater fluxes over the world ocean can also be a very potent factor in THC dynamics. To address the role of changes in both high-latitudinal and inter-basin freshwater fluxes in altering the global THC, we performed several simple numerical experiments. First, we ran the atmospheric control experiment using the NCAR Community Climate Model (CCM) with observed sea surface temperature (SST) and salinity to get the present-day control atmospheric state, that is, the wind stress, SST, and freshwater fluxes across the sea surfaces. Next, we ran the oceanic control experiment using the GFDL Modular Ocean Model (MOM) with these sea surface conditions from the CCM. In the first series of experiments, we specified idealized anomalies of freshwater fluxes in the northern North Atlantic, the Southern Ocean, and the subtropical North Atlantic and North Pacific. These experiments gave us insight on the relative importance of high-latitudinal and inter-basin short-term fluctuations in freshwater balance for the THC dynamics. In the second series of experiments, we simulated the disruption of the freshwater regime in the northern North Atlantic caused by freshwater floods from Lake Agassiz (a glacial lake that drained into the Hudson Bay around 8.2 kyr BP). The estimates of freshwater discharged into the northern North Atlantic suggest that these volumes of freshwater could be sufficient for large-scale disturbances of the THC. The results of the two series of experiments will be shown and discussed.
http://www.essc.psu.edu/~dseidov/
PP44A-03 16:30h
Glacial ITCZ Shifts Recorded by Tropical Salinity Reconstructions in the Pacific and Caribbean
A variety of data from terrestrial, marine and modeling studies indicate that the tropical Atlantic and Pacific ITCZ was shifted considerably south of its current position during the last glacial period. The effect of such a shift on the glacial ocean would be to increase surface salinity (relative to today) in regions that are currently influenced by the ITCZ and to decrease salinity in regions that are currently south of the modern ITCZ but within the influence of the glacial ITCZ. Here we compare a previously published 380 ka \delta$^{18}$O$_{sw}$ reconstruction (a proxy for salinity) for the western equatorial Pacific (WEP) at ODP Site 806B ($0.33\deg$N, $159.34\deg$E) (Lea et al. 2000) with a new Caribbean \delta$^{18}$O$_{sw}$ reconstruction at ODP Site 999A ($12.75\deg$N, $78.75\deg$W) that was derived from Mg/Ca (a proxy for SST) and \delta$^{18}$O data from a surface dwelling foraminifera, {\it Globigerinoides ruber}. These reconstructions indicate the WEP was considerably fresher during past glacial periods with surface \delta$^{18}$O$_{sw}$ 0.3 to 0.4 \permil lower than today after removal of ice volume. Using modern \delta$^{18}$O$_{sw}$ vs salinity relationships, this geochemical change suggests glacial surface salinities in the WEP at Site 806B were ~1-1.5 psu lower than today. In contrast, Caribbean surface waters were much saltier during the last glacial period with \delta$^{18}$O$_{sw}$ 0.5 \permil higher than modern after removal of ice volume. This \delta$^{18}$O$_{sw}$ shift translates to a glacial surface salinity $>$2 psu higher than today (after the influence of ice volume was removed). Reconstruction of surface salinities across 3 glacial cycles back to 380 ka at these two sites demonstrate that the Caribbean and WEP were antiphased with large salinity differences during glacials and nominal differences during interglacials. Tropical western Pacific sediment trap data indicate the maximum {\it G. ruber} flux is during the summer months. Hence, we interpret these data to indicate that the glacial summer ITCZ was located over the equator in the WEP, considerably farther south than its current position at $8-10\deg$N. In contrast, the elevated Caribbean salinities support terrestrial and coastal Cariaco Basin data that indicate the ITCZ was situated over the over South America during the summer months. We are optimistic that future \delta$^{18}$O$_{sw}$ records from adjacent tropical sites will permit us to reconstruct the continuous migration of the ITCZ through time.
PP44A-04 16:45h
Sea-surface hydrographic variations during the past 170 thousand years in the southeastern South China Sea
Using Mg/Ca ratios of planktonic foraminifer {\it Globigerinoides sacculifer} and alkenone unsaturation index of bulk sediments from the deep-sea core MD972142, we reconstructed sea-surface temperatures of the past 170 thousand years (kyrs) for the southeastern South China Sea (SCS) near the Palawan Island. The SSTs fluctuated between ~$24\deg$C and $29\deg$C through the past two glacial-interglacial cycles with the coldest temperature of $24\deg$C at the last glacial maximum. The SSTs during the late Holocene are comparable to the peak values of the marine oxygen isotope stage 5. The effects of temperature and ice-volume were subtracted from the $\delta$18O values of planktonic foraminifer {\it Globigerinoides sacculifer} to examine the past salinity and precipitation variations. The resultant residual $\delta$18O profiles indicate that the surface oceans have been fresher than today's for most of the time during the past 170 kyrs. In general, during the low sea-level periods, the "ice-volume-corrected sea water $\delta$18O" values tend to deviate from the current value, signifying various degrees of freshening of sea surface waters. Contrarily, during high sea-level stands, the surface waters tend to be as salty as it is today. A detailed examination of the hydrographic variation during the past 22 kyrs shows that the ice-volume-corrected $\delta$18O varied by 1 $\permil$. The relatively light values during the last glacial maximum and the Younger Dryas are ascribed to the low sea-level and thus a limited water exchange between the open ocean and the SCS. The Holocene values are virtually the same as today's except for two minor increases during 8-9 ka and ~ 3 ka. Notably, these two "salty" events are coincident with the "8.2 ka" event and the "{\it Pulleniatina} Minimum Event". A reduction of precipitation and thus a weakening of the summer monsoons might be the regional manifestation of these two events in the South China Sea.
PP44A-05 INVITED 17:00h
The Inter-Ocean Salinity Gradient-Holocene vs. Glacial
In a previous investigation of LGM and Holocene surface water $\delta$$^{18}$O$_{SMOW} ($\delta$w)and temperature variability within the western tropical Pacific we documented higher $\delta$w values ($\sim$0.5% higher than the ice volume component) during the LGM and during stadials, implying increased salinities at times of tropical cooling (Stott et al., 2002). We have also recently documented a systematic decrease in SSTs and $\delta$w during the late Holocene between 8$^{o}$N and 10$^{o}$S in the western tropical Pacific that indicates surface salinities have decreased progressively by about 1psu within the Pacific Warm Pool since the early Holocene (Stott et al., 2004). The progressive decrease in western Pacific salinities during the Holocene tracked shifts in the ITCZ in response to changing orbital forcing. We hypothesized that shifts in the ITCZ during millennial-scale climate shifts disrupted the exchange of moisture between the Atlantic and Pacific and affected the salinity contrast between the two ocean basins. If so, it is possible that the hydrologic cycle has had a primary role in modulating thermohaline circulation during large climate oscillations. However, on the basis of the Pacific records alone we can not assess whether the trends we observed in the Pacific were accompanied by opposing changes in the Atlantic. In this study combined planktonic foraminiferal oxygen isotope and Mg/Ca paleothermometry to reconstruct the oxygen isotope composition of tropical surface waters ($\delta$w) in the Atlantic during the last Glacial and Holocene in order to assess whether there has been a redistribution of salt between the two oceans over last 20 thousand years. We focus on the subtropical regions of the tropical Atlantic and the western tropical Pacific in this comparative study because these two regions should be sensitive to changes in the vapor exchange between the two basins. Diminished freshwater exchange between the Atlantic and the Pacific when the ITCZ is in a more northerly position would tend to decrease salinities in the Atlantic and increase them in the Pacific. Samples of Globigerinoides ruber (white) were taken every centimeter through 8 box cores collected at sites along the mid-Atlantic ridge between 20$^{o}$N and 20$^{o}$S. The carbonate sequences contain well preserved foraminifera but the sedimentation rates at these locations are low ($\sim$ 1.5cm/ka). However, we showed in a previous study that despite the low sedimentation rates, the amount of mixing has not significantly effected the oxygen isotopic contrasts between glacial and Holocene samples (Stott and Tang 1996). Therefore, although it is not possible to resolve discrete millennial-scale features from the geochemical records, it is possible to resolve whether there has been a systematic shift in the dw and hence salinity. In the subtropical North and South Atlantic G. ruber Mg/Ca temperatures were 1.5$^{o}$cooler during the LGM compared to the late Holocene. We observe that at each of the sites within the subtropical north and south Atlantic the Holocene-LGM dw contrast averages 1%, which is equivalent to the ice volume effect. On the basis of these records we conclude that there was not a systematic change in the salinity of the mid ocean gyres in the North and South Atlantic during the LGM. Furthermore, these data do not support the hypothesis that shifts in the ITCZ during the LGM and during the Holocene affected the salinity contrast between the two ocean basins.
PP44A-06 17:15h
Gulf Stream Salinity Variation During MIS 3 and its Link to D-O Cycles
Paleoclimate archives indicate that Marine Isotope Stage 3 (MIS 3) was characterized by a highly variable climate, expressed in the Greenland ice cores by the Dansgaard-Oeschger (D-O) fluctuations associated with large temperature changes (Johnsen et al., 2001). However, the question remains as to the trigger for these abrupt Northern Hemisphere climate swings. A popular theory implicates thermohaline circulation instability as a driver for D-O cycles, suggesting that salinity variability in surface waters delivered to the sub-polar North Atlantic via the Gulf Stream may have played a role in driving millennial-scale oscillations in glacial thermohaline circulation (Broecker et al., 1990; Zaucker and Broecker, 1992). In order to investigate the relationship between Gulf Stream salinity variation and D-O cycles, we combine Mg/Ca measurements (a proxy for the temperature of calcification) with ($\delta$$^{18}$O) analyses of shells from the surface-dwelling foraminifera {\it Globigerinoides ruber} (white var.) from ODP site 1060 located beneath the Gulf Stream on the Blake Outer Ridge (36.77$\deg$N, 74.47$\deg$W; 3480 m; 20-53 cm/kyr sed. rate) to produce a high-resolution record of $\delta$$^{18}$O$_{SEAWATER}$ ($\delta$$^{18}$O$_{SW}$) during MIS 3 (49.5- 59.2 kyr, corresponding to Interstadial 13 to 16 on the SSO9sea time scale). The Mg/Ca-temperature (SST) record from 1060 shows minimal variability ($\pm$3$\deg$C) between stadials and interstadials. Average interstadial Mg/Ca-SSTs (25.2$\deg$C) agree with faunal August SST reconstructions (Vautravers et al., 2004), but stadial Mg/Ca-SST reconstructions are warmer than faunal estimates. The calculated $\delta$$^{18}$O$_{SW}$ record co-varies with the Greenland ice core $\delta$$^{18}$O record (Johnsen et al., 2001), and indicates high stadial salinities, similar to MIS 3 Caribbean salinity reconstructions (Schmidt et al., 2004), followed by abrupt decreases in $\delta$$^{18}$O$_{SW}$ up to 0.9$\permil$ occurring in less than 250 years on the transition to interstadials. We hypothesize that the input of isotopically light freshwater from melting North American ice sheets during interstadials accounts for the rapid, large decrease in Gulf Stream $\delta$$^{18}$O$_{SW}$. Our $\delta$$^{18}$O$_{SW}$ reconstruction from ODP 1060 therefore suggests that D-O cycles directly impact Gulf Stream salinity during MIS 3, with elevated salinities occurring during stadials followed by a rapid decrease of salinity into interstadials in response to Northern Hemisphere warming.
PP44A-07 17:30h
Evidence for Oceanic/Continental Climate Linkages During Freshwater Inputs to the Gulf of Mexico
Understanding the linkage between oceanic and continental responses to millennial-scale climate variability can provide insight into the mechanisms controlling abrupt global climate change, however there are few marine depositional systems where these linkages can be studied simultaneously. Here we present new evidence for freshwater input (Laurentide Ice Sheet (LIS) meltwater versus precipitation?) to the northern Gulf of Mexico from 28-45 ka (within Marine Isotope Stage 3), and draw linkages to continental climate using organic geochemical proxies. A 32-m laminated sediment core (MD02-2551) from the anaerobic Orca Basin was collected aboard the R/V Marion Dufresne in July 2002 as part of the IMAGES program. Radiocarbon dates suggest that the average sedimentation rate is $>$50 cm/1000 years during this interval, allowing for 40-year resolution sampling at 2-cm intervals. Paired $\delta$$^{18}$O and Mg/Ca data on the planktic foraminifer Globigerinoides ruber are used to separate changes in Mg-derived sea-surface temperature (SST) and the oxygen isotopic composition of seawater, which can be interpreted in terms of salinity. Three large negative excursions in $\delta$$^{18}$O$_{sw}$ ($<$0.5 \permil), each lasting ~3 ka, exist in the record. Two of these excursions may correlate with Heinrich events 3 and 4, but show no clear relationship to Dansgaard-Oeschger warmings. $\delta$$^{18}$O$_{sw}$ excursions have significant ramifications for Gulf of Mexico salinity, which can be constrained using two freshwater end members: the current Mississippi River (MR) value of -7 \permil and an LIS value of -30 \permil. Use of the MR end-member would require extraordinarily large floods and changes in salinity of 5-6 psu, whereas use of the LIS end-member results in a salinity decrease of 2-3 psu. Preliminary results of analyses of organic biomarkers preserved in the Orca Basin sediments suggest that the freshwater events are dominated by terrigenous organic matter, but are also associated with an increase in marine-derived algal inputs, suggesting that changes in the nutrient flux to the Gulf of Mexico from the North American continent may support marine production. Molecular carbon and hydrogen isotopic components of biomarkers will be used to evaluate shifts in continental climate through analyses of vegetation ($\delta$$^{13}$C; C3 vs C4 plants) and source water ($\delta$D) changes during large salinity excursions observed in the Gulf of Mexico.
PP44A-08 17:45h
Florida Current Temperature and Salinity Variability during the Last Millennium
Planktonic foraminiferal time series from four well-dated, high sedimentation rate cores exhibit significant changes in Florida Current $\delta^{18}$O, SST, and $\delta^{18}$O$_{w}$ during the past 1000 years. On the Florida Margin (24.4 $\deg$N, 83.3 $\deg$W), {\it G. ruber} $\delta^{18}$O in two cores increased by 0.1-0.2 $\permil$ from 500 to 200 yr BP. During the same time interval, Mg/Ca-derived SSTs warmed by 0.5-1.0 $\deg$C. These shifts in $\delta^{18}$O and temperature require an increase in $\delta^{18}$O$_{w}$ of 0.2-0.4 $\permil$, equivalent to a salinity increase of 1-2 psu. From 200 yr BP to present, $\delta^{18}$O$_{w}$ values decreased by 0.1-0.2 $\permil$. On the other side of the Florida Current (FC), two cores from the Great Bahama Bank (24.6 $\deg$N, 79.3 $\deg$W) also support a ~1-2 psu increase in salinity, but the shift occurs about 200 years later. {\it G. ruber} $\delta^{18}$O at the Great Bahama Bank sites changed little from 1000 to 300 yr BP, but from 300 yr BP to present, it increased by 0.2-0.3 $\permil$. SSTs during the last 300 years increased by 0.5-1.0 $\deg$C, indicating that $\delta^{18}$O$_{w}$ increased by 0.3-0.4 $\permil$ since 300 yr BP. In the earlier portion of the record, $\delta^{18}$O$_{w}$ variability is on the order of 0.1 $\permil$. Mg/Ca analyses from the Great Bahama Bank indicate that coretop SSTs are similar to those at 1000 yr BP (the so-called Medieval Warm Period). SSTs were 0.5-1.0 $\deg$C cooler during the Little Ice Age (~750 to 200 yr BP). Although small, these changes in SST are consistent in magnitude and timing in two separate time series. The Florida Current SST variability is similar to that estimated for the Sargasso Sea (Keigwin, 1996), although the Sargasso Sea reconstruction implies higher SSTs than modern during the Medieval Warm Period. Changes in Florida Current salinity result from evaporation-precipitation variability in the central and western tropical Atlantic, the source region for much of the FC surface water (Schmitz and Richardson, 1991). Evidence of centennial-scale hydrologic variability from the Cariaco Basin (Haug {\it et al.}, 2001; Black {\it et al.}, 2004) and the Yucatan Peninsula (Hodell {\it et al.}, 2001; 2002) implies the Inter-Tropical Convergence Zone (ITCZ) shifted southward during the Little Ice Age. If the ITCZ migrated south during the LIA, it may have led to increased salinity in the tropical Atlantic, Caribbean, and the FC. While modern observational records indicate the salinity of the Florida Margin sites follows that of the Caribbean and tropical Atlantic, the Great Bahama Bank is more influenced by the relatively salty North Atlantic subtropical gyre (Levitus, 1994). The time lag in the $\delta^{18}$O$_{w}$ records from the Florida Margin and Great Bahama Bank indicates that salinity first increased in the tropical Atlantic, followed by a similar magnitude increase about 200 years later in the North Atlantic subtropical gyre.