PP13D-01
A Mid-Latitude Atmospheric Trigger of the Bipolar Ocean See-Saw?
Since the search remains elusive for an oceanic trigger for the bipolar ocean see-saw, the attention of some has shifted to the atmosphere. In particular, the tropical Pacific has been suggested as a possible, even likely, source for an atmospheric trigger. Here we discuss results from a very high-resolution (T170, ~75 km grid) GCM simulation of the last glacial maximum that suggests that the mid-latitude atmospheric circulation, rather than being just a conduit from the tropics, may also have an internal variability that could be a contributing factor, or perhaps even a substitute for a tropical trigger. Preliminary results suggest that the interannual variability in the model glacial atmosphere is greater than for the control run. The model also has intraseasonal variability that can lead to a °C change in temperature over central Greenland in three days, i.e., the time scale of a standard synoptic-scale weather system in mid-latitudes. Because the simulation involved prescribed sea surface temperatures (ie, no ocean variability), the variability in the model winds must be internal to the atmosphere. Examples will be given from model simulations and a movie as to the hourly-scale mid-latitude meteorological changes that accompany rapid temperature changes over Greenland.
PP13D-02 INVITED
231Pa/ 230Th as a Paleocirculation Proxy in the Southern Hemisphere
Since its first application as a proxy for the rate of past ocean circulation more than a decade ago (Yu et al.
1996), 231Pa/ 230Th has been applied predominantly to cores in the North Atlantic. At present, in
this setting, the advection of surface waters to depth resets the 231Pa/ 230Th "clock" allowing the
proxy to work, and the uniform southward movement of water masses make interpretation of sediment values
relatively straightforward.
The applicability of 231Pa/ 230Th in other settings, where water-masses may not be clearly reset,
or where several water masses with different flow paths overlie one another remains unclear, but its use to
reconstruct past flow rates in such settings would provide a powerful tool to understand past climate.
We will present an overview of the potential for 231Pa/ 230Th use in the Southern Hemisphere,
including new data from the Argentine basin. Data from the southern Indian Ocean (e.g. Thomas et al. 2007)
demonstrate that advection of water to depth, and scavenging of 231Pa at the opal belt, effectively
reset the 231Pa/ 230Th clock providing potential for application of the proxy to northward flowing
water masses in the Southern Hemisphere. Modelling also indicates the sensitivity of Southern Hemisphere
sediment 231Pa/ 230Th to changes in ocean circulation, particularly in the Argentine Basin (Siddall
et al. 2007). We have investigated the use of the 231Pa/ 230Th proxy in four cores spanning all
deep-water masses in that basin. The modern-day conditions in that basin are similar to those expected in
the North Atlantic during the last glacial maximum with a southward and northward flowing water mass
overlying one another. Results demonstrate that sediment 231Pa/ 230Th values vary significantly
with water-mass throughout the last 20 kyr – offering the potential to reconstruct multiple water masses
through time. Down-core results show variations that reflect changes in ocean circulation, but these are
compounded with other effects, probably due to changing boundary scavenging as sea-level changes.
Approaches to deconvolve these effects and to use 231Pa/ 230Th as an accurate proxy for
Southern Hemisphere deep-water flow will be presented.
E.F. Yu, R. Francois, and M. Bacon, Nature 379, 689 (1996).
A. L. Thomas, G. M. Henderson, and I. N. McCave, Paleoceanogr. 22 (4) (2007).
M. Siddall, T. F. Stocker, G. M. Henderson et al., Paleoceanogr. 22 (2) (2007).
PP13D-03
Multi-Proxy Records for Testing the Role of Indian-Atlantic Gateway Circulation and Inter- Ocean Exchanges in Modulating the Atlantic MOC
Transient behaviour of the ocean MOC traditionally is viewed from the perspective of North Atlantic perturbations. Buoyancy forcing in connection with anomalous freshwater perturbation near the centres of deep convection in the North is assumed a prime driver of MOC shifts, a contention supported by a body of palaeo-evidence from the region. Recent concepts, however, increasingly put the focus on the southern hemisphere oceans as players in the global ocean MOC as palaeo-profiles from the South display at least as much variability as do those from the North Atlantic. New profiles from the Agulhas Corridor off South Africa add to this picture in that they display an atypical structure alluding to the significance of Agulhas Current transports in setting the hydrography in the Indian-Atlantic ocean gateway. Planktonic Mg/Ca and δD (from alkenones) suggest a build-up of warm/salt waters in the gateway during glacials. Benthic δ13C and Cd/Ca indicate variable advection of southern and northern hemisphere deep water components during glacial periods, some (but not all) in conjunction with buoyancy forcing in the North while, SS and 231Pa/230Th document an enhanced physical circulation in the South during the glacials. Connecting the surface ocean data with the deep water palaeo-records suggests the interplay between buoyancy forcing in both hemispheres acted to modulate the MOC, rather than forcing in the north alone. Notably abrupt shifts in salt water prominence in the Indian-Atlantic gateway that are indicated in the palaeo- records in connection with major MOC shifts strengthen the case that the southern hemisphere oceans contributed crucially in the past to the state of the global MOC and climate, and will play their role in future developments.
PP13D-04
Quantifying Late Quaternary Changes in Thermohaline Circulation With Nd and Pa-Th Isotopes
Nd isotope and 231Pa/230Th ratios are increasingly used as paleocirculation tracers. Both tracers show promise to constrain transport rates, and thus quantify past changes in the global thermohaline circulation (THC). Nd isotopes act as "passive" tracers of the water mass mixture, while and Pa/Th ratios act as "dynamic" tracers of export flux. For Nd isotopes, THC vigor is reflected by changes in the Circum-Antarctic, which connects the three major ocean basins and acts as the intermediary of global water-mass exchange. For example, weakening export of NADW or GNAIW would result in weaker in North Atlantic signatures in the deep Circum-Antarctic, and a shutdown might be reflected by Nd isotope ratios like the Pacific. For Pa/Th, a shutdown might be reflected by 231Pa/230Th ratios that approach the production ratio in the North Atlantic. Because Nd isotopes and Pa/Th are controlled by fundamentally different processes, agreement between these proxies can be taken as corroboration that records reflect THC changes. We present a 30 kyr Nd isotope record representing the Circum-Antarctic "global blend" and allowing a first order quantification THC variability. The core, V19-188, from the deep Indian Ocean and bathed in CDW, shows coretop εNd of -8, in agreement with regional seawater and consistent with Circum-Antarctic provenance. The record indicates large variations THC intensity. A conspicuous feature is Pacific-like Nd isotope ratios during Heinrich Event 1 (H1) and the early part of MIS 2 at >23 kyr (encompassing the intervals of H2 and H3), indicating absence of any North Atlantic-derived component and implying shut-down of the global THC during these intervals. The Younger Dryas also imparts a significant but smaller magnitude THC weakening. However, the THC is moderately strong during the LGM around 20 kyr. The record is strikingly similar to the North Atlantic Pa/Th record of McManus et al. (Nature 2004), from the Holocene through the LGM at ~20 kyr, where that record ends. We suggest that further comparison Nd isotopes representing the Circum-Antarctic and and Pa/Th representing the North Atlantic can be an important means of quantifying THC intensity and its impact on climate.
PP13D-05
Long- and Short Term Holocene Variations in the Strength of the North Atlantic Deep Water Revealed by the Analysis of the Sedimentary Magnetic Properties.
The precise documentation of the natural variability of the THC during the interglacials in North Atlantic is of critical interest for the understanding of its future evolution under the effect of global warming. We report on detailed magnetic analyses of the entire Holocene period from several marine sequences distributed from northern to southern Gardar and Bjorn drifts (south of Iceland, 53°N to 61°N) and the Charlie- Gibbs fracture zone (52°S) taken in 1977 with the R.V. J. Charcot from IFREMER and on board the R. V. Marion Dufresne during the MD132-P.I.C.A.S.S.O cruise in 2003. The average sedimentation rate of these sequences varies during the Holocene between 20 and 80 cm/kyr. The magnetic fraction is uniformely composed during the Holocene period of magnetites with a grain size distribution in the pseudo-single domain range (a few micrometers). The magnetic concentration varies in space and in time on both long and short terms. In space, from North to South, the amount of magnetite decreases, together with the magnetic grain size along the Gardar and Bjorn drifts illustrating a progressive deposition of the magnetic fraction derived from the basaltic Iceland-Faeroe province along the path of the Iceland-Scotland overflow water branch of the NADW. In turn, this proves the ability of the magnetic properties to detect and trace rapid oceanic circulation changes. In time, long-term variations in the amount of magnetites transported to the studied sites from the northern basaltic province are not quantitatively compensated by changes in the carbonate fraction. The long-term trends may be fitted by a 4th order polynomial with an increase of the bottom circulation strength in the mid-Holocene. We'll discuss these long term trends together with the short term features identified for the sequences dated by 14C in terms of changes in the strength of the bottom current through Holocene and compare them to other proxies an other records also obtained from North Atlantic.
PP13D-06 INVITED
On the Abyssal Circulation in the Atlantic Basin at the Last Glacial Maximum
Our understanding of oceanic variability on time scales longer than the time span of direct oceanographic measurements (about a century at best) relies on our capability to interpret the marine sediment record. Sediment observations have reached the point where some general hypotheses about ocean circulation during the Last Glacial Maximum (LGM, near ca. 20 kyr B.P.) can be tested. Among the most common observations are δ18O and δ13C on benthic foraminifera. A recent compilation of δ13C data has been taken to imply the existence in the glacial Atlantic of a southern source water near 1000 m water depth, a northern source water near 1500 m, and a southern source water below ca. 2000 m (Curry and Oppo, 2005). Here an inverse method is used to provide a rigorous test of the null hypothesis that δ18O and δ13C data for glacial sediments are consistent with the modern circulation. Particular emphasis is put on the minimal uncertainties in the sediment data and in our current understanding of the behavior of the oxygen and carbon isotopes in the ocean, which must be assumed in order to rule out the hypothesis with confidence.
PP13D-07
Holocene Oscillations in the Temperature and Salinity of the Surface Subpolar North Atlantic
The Atlantic meridional overturning circulation (AMOC) transports warm and salty surface waters to high latitudes (via the North Atlantic Current, NAC), where they cool, sink, and return southwards at depth. Through its attendant meridional heat transport, the AMOC helps maintain a warm NW European climate, and acts as a control on global climate. Yet our ability to test hypotheses about AMOC behavior during periods of climate change is limited by the short time period for which instrumental data is available. To address this problem, we reconstruct the temperature and salinity of the NAC using paired δ18O-Mg/Ca measurements of foraminifera from ocean sediment core RAPiD-12-1K, located on the South Iceland Rise, throughout the Holocene (0-11.7 ka). The records are interpreted by examining the modern controls on the hydrography of the region. The paleo data then in turn provides support for recent modeling studies of AMOC behavior, thus providing an integrated view of ocean dynamics. G. bulloides data reveal millennial timescale salinity variations (~0.5 psu) superimposed upon a trend of increasing near-surface water salinity from ~9 ka to the present. The shorter timescale variability is likely due to southward advances of the subpolar front, analogous to the changes observed during the NAO minimum during the 1960s. The long term trend in near surface salinity may be caused by shifts in the ITCZ and the input of deglacial meltwater. G. inflata data show that below the seasonal thermocline the NAC has undergone millennial variations in temperature and salinity (~3.5°C and ~1.5 psu). These are controlled by subpolar gyre dynamics, consistent with modern studies of inter-annual to decadal timescale behavior. The inflow becomes more saline during enhanced freshwater flux to the subpolar North Atlantic, suggesting a negative feedback is in operation on millennial timescales. The AMOC may therefore be more stable than previously expected during future global warming. Near-surface and sub-thermocline data are combined to produce upper water column density stratification records. The records show a stratified upper ocean during the early Holocene with an abrupt switch to well mixed waters at ~8.4 ka, followed by quasi-periodic stratification events every ~1500 years. This suggests that surface circulation was fixed in one mode of operation prior to ~8.4 ka, perhaps due to the deglacial input of meltwater to the SPG. Later, with reduced freshwater input, the system oscillated between two modes of operation, involving strong and weak SPG circulation. This threshold behavior is similar to that displayed by recent modeling studies.
PP13D-08 INVITED
Physical and chemical evidence for past variations in bottom waters along the Reykjanes Ridge south of Iceland
Nick McCave pioneered the sortable silt approach to determining the physical sorting by currents that allows grain size distributions to be applied as a dynamical proxy of past bottom current strength. He also championed the multi-proxy approach to paleo-reconstructions by showing that measurements of physical parameters provide complementary information to that of chemical and isotopic measurements. The region of deep overflows from the Nordic Seas over the Greenland-Iceland-Scotland Ridge, and the resulting bottom currents along the Reykjanes Ridge, provides a natural laboratory to explore the deep and intermediate circulation of the North Atlantic. McCave's conclusions about changes in the strength of these flows between the last ice age and today continue to stand up after more than a decade of sustained subsequent research into this oceanic circulation system. We have returned to the Reykjanes Ridge study area for a high-resolution multi-proxy study of the last deglaciation. Using a depth transect covering intermediate to deep waters, we can identify water mass changes and the bottom current strength associated with them. The highest resolution site, ODP 984, sits at 1650 meters water depth along a different axis of flow from the other core locations. Records from this location clearly show the presence and vigorous flow of a high d13C intermediate water mass at the last glacial maximum (LGM), supporting McCave's original observations and inferences. The higher resolution record also captures repeated deglacial declines in both carbon isotopes and sortable silt mean grain size associated with abrupt climate change events including the Heinrich event iceberg discharges and Younger Dryas regional cooling. Cores from the depth transect display consistent patters of d18O and d13C indicating the dominance of a glacial intermediate water mass at this location, and a strong shift to low d13C waters early in the deglaciation. The mean sortable silt grain size at several of the intermediate sites increases from the LGM, possibly indicating a more complicated flow regime along the Rekjanes Ridge than evident at any single study site.
PP13D-09 INVITED
Altered Circulations: Contrasts Between Modern and Glacial N. Atlantic and S.W. Pacific.
Palaeohydrographic and flow speed data suggest that water mass boundaries shifted profoundly from the glacial to modern N. Atlantic. Southern source water (SSW) occupied the basin below 2 to 2.5 km depth. Northern source water (NSW) flowed strongly along western (and northern) boundaries above that depth. Flow of SSW appears to have been modulated by climatic shifts in the Southern Ocean. In the S. Atlantic sector of the Southern Ocean the depth of 2.5 km marks the boundary between well ventilated water above, probably NSW supplied by the vigorous boundary current, and high nutrient (or poorly ventilated) (HN/PV) water below down to > 5 km. By contrast, water mass boundaries in the Pacific deep western boundary current (DWBC) east of New Zealand do not appear to have changed significantly between glacial and interglacial with Lower Circumpolar Deep Water (LCDW) below ~3.5 km, Antarctic Intermediate Water (AAIW) above ~ 1500 m, and an HN/PV water mass in between. The major change in circulation here is that the HN/PV water mass presently comes from the north as North Pacific Deep Water whereas at the LGM it came from the south, but only shallower than 3.5 km, not over the full depth to 5 km. Below 3.5 km the glacial Southern Ocean was segmented by barriers, with the Ross Sea supplying to the Pacific the water mass that is presently LCDW. The flow strength of the DWBC does not appear to have changed much from glacial to interglacial, a situation apparently similar to that in the S.W. Indian Ocean DWBC.