PP42B-01 INVITED
Multi-proxy evidence for the behaviour of the Atlantic Meridional Overturning circulation in the Neogene
Though the overall pattern of the modern deep-water circulation in the Atlantic was probably established in the Neogene, there is also increasing evidence suggesting significant secular variation over the past 10-15 Ma forced by an array of potential processes. These include continued sill development in the North Atlantic, the advent of sea-ice in the Southern Ocean and the closure history of the Central American Seaway. Deciphering the relative importance of these different processes is not always straightforward using traditional circulation proxies, such as those based on deep-water nutrient distributions. For example, changes in the nutrient or 13C inventory of deep water in the Atlantic could track changes in one of three different phenomena: the pattern of mixing between southern- versus northern-derived water masses; the extent of nutrient-utilisation in the surface Southern Ocean; or vertical nutrient cycling at the location investigated. Only a multi-proxy approach to this problem can unpick these various controlling processes. Nd isotopes provide a relatively new and potentially powerful tracer of deep-water circulation in the Atlantic, and the deep water Nd isotope composition may be extracted at high time resolution from the authigenic fraction of deep-sea sediments. Such an approach has been shown to isolate circulation changes from those related to the carbon cycle and nutrient-cycling in the Late Quaternary1. We have already published2 combined records of foraminiferal Cd/Ca ratios and Nd isotopes for the Neogene of the deep South Atlantic (ODP Leg 208, site 1264). Here we present a higher resolution (up to 100 kyr) version of this dataset. In combination with studies of secular δ13C gradients3 down the deep Atlantic, these data help to clarify the processes that control deep water chemistry and isotope geochemistry. For example, the Nd isotope data eliminate ambiguities in the carbon data surrounding the relative importance of the pattern of deep water mass mixing (south versus north) versus changes in the source water masses themselves (e.g. sea-ice growth in the South or seaway history in the North). Together the data suggest NADW export to the South Atlantic at 3-4 Ma that is as strong as today. Prior to that Nd isotopes also imply relatively strong NADW export, while a relative lack of coherence between the nutrient-based proxies and Nd isotopes may record changes in the pre-formed nutrient characteristics of southern-derived water masses. 1Piotrowski, A.M. et al. (2005)Science 307, 1933-1938. 2Klevenz, V. et al. (2008) EPSL 265, 571-587. 3Hodell, D.A. and Venz-Curtis, K.A. (2006) Geochem. Geophys. Geosys. 7, Q09001, doi:10.1029/2005GC001211.
PP42B-02
Mid-Pliocene Changes In MOW Recorded East And Northwest Of Gibraltar
Mid-Pliocene deep-water temperatures (DWT) and δ18Odw salinities at northeast Atlantic DSDP Site 548 (1250 m w.d., west of Brittany) are compared with records from West Mediterranean ODP Site 978 in the Alboran Sea (1930 m w.d.). Mediterranean Outflow Water (MOW) was traced by high εNd values of -8.5 to -10. Millennial-scale records of MOW composition were derived from Mg/Ca and δ18Odw values in epibenthic foraminifera for MIS Gi5/Gi1 to MIS G6/101, 3.68/3.62 - 2.72/2.56~Ma (tuned to age scale LR04). West Mediterranean DWT oscillated at a fairly constant level near 14-18°C, 3°C higher than today, which implies ongoing convection of warm Mediterranean deep- water. By contrast, δ18Odw shows a distinct 0.6~‰ increase equal to plus 1.2~‰ salinity near 3.5-3.3 Ma (MIS MG7-MG1), which suggests enhanced salinity at mid-Pliocene cells of Mediterranean deep-water convection, a record of increased Mediterranean aridity. The high-salinity level was traced until 2.7~Ma (MIS G6). A marked mid-Pliocene salinity increase 1.5~‰ also occurred to MOW spilling the northeast Atlantic Site 548 over the interval studied. Coeval MOW temperatures at Site 548 increased from 6-8°C up to a plateau of 8-11.5°C, which lasted until 2.95~Ma (MIS G17). Accordingly, we may conclude that increased aridity induced a major increase in MOW advection and in turn, an enhanced salt discharge to North Atlantic intermediate waters 3.5-3.3~Ma. This salt, in turn, may have strengthened North Atlantic THC and thus possibly preconditioned the onset of major Northern Hemisphere Glaciation (NHG). In contrast to Alboran Sea Site 978, both MOW temperatures and salinity at northeast Atlantic Site 548 near 2.95-2.82~Ma display a unique but short-lasting drop by 6°C and 3~psu, respectively, a drop coeval with the final closure of the Central American Seaways (CAS) and the onset of major NHG (Bartoli et al., 2005). Later on, northeast Atlantic MOW temperatures and salinities increased by 4-5°C and 0.5-1.5~psu back to levels characteristic of Late Pleistocene-to-modern MOW. Accordingly, high salt discharge from the Caribbean may then have replaced the role of high Mediterranean salt input for Atlantic THC subsequent to the final closure of CAS. References: Bartoli, G., Sarnthein, M., Weinelt, M., Erlenkeuser, H., Garbe-Schönberg, D., Lea, D.W., 2005. Final closure of Panama and the onset of northern hemisphere glaciation. EPSL 237, 33-44.
PP42B-03
Reconstruction of Pleistocene Climate for the Past Million Years From Benthic Foraminiferal Deep-sea Proxy Records From the Southern Ocean
One of the most intriguing aspects of the Pleistocene climate is the development of 100 kyr glacial variability during the middle Pleistocene. Very little is known about how deep-water temperatures evolved during this climate transition, how it correlated to the surface water variability and how it links with the records of greenhouse gases and atmospheric temperature from EPICA Dome C. We present detailed records of Mg/Ca, Sr/Ca, B/Ca, Li/Ca and stable isotopes on the infaunal benthic (Uvigerina spp.) foraminifer over the time interval 0-1 Ma from a marine sediment core recovered in the mid Southern latitudes (ODP Site 1123, Southwest Pacific Ocean). Associated records for planktonic (Globigerina bulloides and Globorotalia inflata) foraminifera have also been obtained. The Mg/Ca record has been assessed for secondary effects additional to temperature and comparisons made with ancillary trace element data. The Mg/Ca-based water temperature estimates allow us to examine the deep ocean response to glacial-interglacial changes in the atmosphere and surface ocean over this critical period and, using the paired foraminiferal O isotope and Mg/Ca records explore the changes in seawater O isotopic compsition.
PP42B-04
Influence of Agulhas Leakage on the Meridional Overturning Circulation (MOC): Insights From Paired Planktonic and Benthic Foraminifera Stable Isotope and Trace Metal Analyses Over the Last 345 kyr
The Agulhas Current off South Africa plays a key role in the Atlantic MOC as salty and warm Indian waters are transported to the Atlantic Ocean and influence its buoyancy budget. Its importance is also highlighted in paleoceanographic data profiles and numerical models that suggest that increased water transports from the Indian to the Atlantic prior to and during glacial Terminations may have contributed to the onset of interglacial conditions. Planktonic δ18O and Mg/Ca-derived SST from the Agulhas Bank Slope off South Africa in combination demonstrate that during glacial periods the presence of Agulhas Waters increased progressively during glacials to maxima values some 5 kyr before glacial Terminations. This is particularly evident during MIS 6 and is further supported by a gradual increase in abundance of the 'Agulhas Leakage Fauna'. Paired benthic δ13C and Cd/Ca during early glacials document a continuous influence of Northern Component Water at depth while the planktonic records indicate a presence of Agulhas Water at levels similar to interglacials, suggesting a persistence of the modern-type linkage between Agulhas Leakage and the MOC. Maximum presence of Agulhas Waters at the surface during late glacial phases coincides with the onset of benthic δ13C and Cd/Ca resumption to interglacial levels. This supports the hypothesis that Agulhas Leakage has contributed to the resumption of the Atlantic MOC to full interglacial vigour. Conversely, during MIS 2 and mid-MIS 6 and 8 increased levels of Agulhas indicators coincide with maximum presence of Southern Component Water at depth. This indicates that at these times either the prominent presence of Agulhas Waters did not stimulate the Atlantic MOC or that other factors, presumably related to North Atlantic climatology as suggested by incursions of IRD in the North, played a prominent role at these times.
PP42B-05 INVITED
Instantaneous and inverse coupling between the South and North Atlantic during the last deglaciation
A multi-proxy approach using a combination of faunal and geochemical techniques is used to constrain variations in the Antarctic Circumpolar Current (ACC) during the last deglacial period (26-10 kyr ago). Planktonic and benthic foraminiferal faunal assemblage counts reveal dramatic variations in the position / geometry and apparent vigour of the ACC which are precisely out-of-phase (within dating uncertainty) with the abrupt changes characteristic of the Northern Hemisphere during this interval. In contrast, mono-specific planktonic foraminiferal Mg/Ca ratios appear to reflect more ambient changes associated with the large heat capacity of the Southern Ocean / South Atlantic. Our results demonstrate a direct link between the abrupt changes associated with major reorganisations of the Atlantic meridional overturning circulation and the more gradual adjustments characteristic of the Southern Ocean.
PP42B-06
A New Sedimentary 231Pa/230Th Record From the South Atlantic: Further Insight Into Atlantic Meridional Overturning Circulation (MOC) Through the Last 45 kyr
The role of the Atlantic MOC and its associated heat and moisture transports is crucial in defining a range of climatically relevant processes including the vigour of the global thermohaline circulation and ocean- atmosphere exchanges. Nutrient-based paleo-circulation proxies (δ13C, Cd/Ca) and water-mass provenance tracers (Nd and mineralogical parameters) suggest the Atlantic MOC at the Last Glacial Maximum (LGM) differed from today with North Atlantic Deep Water (NADW) occupying a smaller fraction of the Atlantic basin (Lynch-Stieglitz et al., 2007). However, these proxies do not provide information about the rate of deep-water advection. Radiogenic isotope ratios such as 231Pa/230Th potentially provide such information due to the differential mobility of both isotopes in the ocean (Yu et al. 1996, McManus et al. 2004, Gherardi et al. 2005, Hall et al. 2006). We present a continuous 231Pa/230Th record from IMAGES core MD02-2594 (34°42.64' S; 17°20.32' E; 2440 m water depth). The site is particularly well suited to trace the southward advection of NADW as it transits the Atlantic-Indian gateway. Due to its proximity to the Southern Ocean the site is also suitable to monitor the northward penetration of Southern Component Waters (SCW) that has varied in conjunction with changing NADW strength. Today site MD02-2594 is bathed by NADW which, according to the benthic δ13C profile, has been the case for the last 13 kyr. 231Pa/230Th results for the Holocene, averaging 0.065±0.007, are in the range of mean North Atlantic ratios and consistent with a large-scale export of Pa from the Atlantic to the Indian and Southern Oceans (Yu et al., 1996; Marchal et al., 2000). Assuming that opal fluxes in the equatorial Atlantic did not obscure the NADW signal, the results suggest a residence time of NADW of ~130 yr in the Atlantic, similar to that of 231Pa in the water column (Henderson and Anderson, 2003). Low 231Pa/230Th ratios of 0.042±0.005 are recorded at this site during glacial Marine Isotope Stages (MIS) 2 and 3 indicating either (i) increased NADW advection or (ii) the dominance of a water mass depleted in Pa, presumably a Pa-scavenged water mass from the Southern Ocean (SCW). Low benthic δ13C and increased benthic Cd/Ca at these times, which are characteristic of a nutrient-laden water mass, support the Southern-Ocean source. Assuming that, as it does today, SCW loses much of its Pa to scavenging within the Southern-Ocean opal belt (Thomas et al., 2007), the glacial 231Pa/230Th ratios suggest a transit time of ~100 yr from the opal belt to the MD02-2594 site, ignoring possible contributions from glacial NADW. This scheme is further supported by complementary proxy profiles measured in the core: δ18O,, 230Th normalised sediment focusing, terrigenous material, opal, chlorins and authigenic 238U.
PP42B-07
231Pa/230Th Ratios in Arctic Sediments Related to Water Depth
Seven records of sedimentary 231Pa/230Th from the Central Arctic (Makarov, Amundsen and Nansen Basins) reveal a strong trend of decreased 231Pa/230Th ratios with increased water depth, which has persisted over the last 35,000 years. In Holocene sediments, shallower (1-1.5 km) cores show average ratios close to the production ratio of 0.093, deeper cores (3-3.5 km) show ratios of ~0.05, and cores between 1.5 and 3 km give intermediate values averaging 0.067. The roughly linear relationship between depth and ratio (slope = -0.019, r2 > 0.9) echoes that recently found in coretops from the South Atlantic (Scholten et al., 2008), suggesting that it is not produced by conditions peculiar to the Arctic basin. Average 231Pa/230Th ratios from deglacial and late-glacial sediments give flatter but still evident linear (r2 > 0.9) depth trends, suggesting that the slope of the relationship may reflect changing oceanographic conditions and particle supply. 231Pa/230Th ratios in shallower cores were lower during the late glacial than during the Holocene, suggesting increased export at that time, possibly related to lowered particle fluxes or changes in particle type. Biogenic opal, a strong scavenger of 231Pa, shows no consistent trend in bulk sedimentary weight percent with either depth or time in samples from our cores. Our results indicate that 231Pa has been exported continuously from the deep Central Arctic over the last 35 kyr.
PP42B-08
Applying the spatial mapping approach to 231Pa/230Th as an overturning proxy
The use of the 231Pa/230Th ratio in deep-sea sediments has been developed and used over the last decade as a proxy for the rate of Atlantic meridional overturning circulation (AMOC). The proxy is based on the known ratio of 231Pa and 230Th production by uranium decay in the ocean, and on the different rates of removal to the sediment of the two isotopes. North Atlantic climate and AMOC are believed to be closely related, and so the 231Pa/230Th proxy has most often been applied to North Atlantic sediments over the past glacial cycle, particularly during periods of abrupt climate change such as the Heinrich 1 (H1) iceberg discharge event. Recent studies have used high-resolution downcore records to interpret AMOC circulation at a single location. Although powerful, this approach cannot always rule out local changes in sediment composition, particle rain rate or other factors influencing the 231Pa/230Th ratio, and therefore may not necessarily reflect the mean behavior of AMOC. Here we combine new and existing 231Pa/230Th data from the Atlantic basin to apply the spatial mapping approach to the 231Pa/230Th proxy. Instead of attempting to reconstruct AMOC at a single site, we use weighted averages of spatially distributed data from the last glacial maximum, H1 and the Holocene in an attempt to examine these three key time periods with respect to the average behavior of the AMOC. This approach greatly decreases the likelihood that the results are biased by variations in factors other than the AMOC, allowing us to examine 231Pa/230Th through time as well as in three- dimensional space. Compilation of existing data highlights key gaps in the spatial coverage and is complicated by the challenge of identifying H1 in all cores. Nevertheless we are able to determine broad spatial patterns and calculate 231Pa budgets where suitable data exists. We show that the minimum net export of 231Pa form the North Atlantic by the AMOC occurred during relatively brief intervals such as H1 rather than during the last glacial maximum.