PP11A-1357
Late Slowdown of Atlantic Meridional Overturning Circulation During the Last Glacial Inception: New Constraints From Sedimentary (231Pa/230Th)
The Atlantic Meridional Overturning Circulation (AMOC) response to northern hemisphere insolation forcing
and North Atlantic surface hydrology is investigated on the period covering the Last Interglacial to glacial
Marine Isotope Stage (MIS) 4, with emphasis on transitions from interstadials to stadials. We provide the first
sedimentary (Pa/Th) profiles over the 130 to 60ka interval, which present significant changes despite the
relatively short half-life of 231Pa (32.1 ka). They are measured in two cores from the western (SU90-11,
3645m) and eastern (MD01-2446, 3547m) basins of the North Atlantic. The (Pa/Th) profiles display a high
degree of coherency with available benthic δ13C and foraminifera summer SST profiles from
nearby reference sediment cores. They do not show any correlation with diatom valves flux and are therefore
interpreted in terms of changes in AMOC export.
Our results show an AMOC drop in parallel with the
enhancement of ice rafted detritus, marking the increased growth and calving of Northern ice-sheets. AMOC
remained vigorous well after each decrease of Northern summer insolation within MIS 5 and is likely driven by
the seasonal meridional insolation gradient. The associated transport of heat and moisture from the tropics
towards the Northern latitudes favoured ice-sheets growth. In return, the ice-sheets dynamics impacted the
AMOC during periods of iceberg discharge and enhanced freshwater input in the North Atlantic. The
associated slowdowns of AMOC export are estimated to be about 40%±15%. Following these
slowdowns, the AMOC quickly recovered its interstadial vigour. The lag of the AMOC response to decreased
Northern summer insolation reduces as the continental ice-sheets increase in size, from about 15ka at MIS
5.5--5.4 transition, 10ka at MIS 5.3--5.2 transition to about 3ka at MIS 5.1--4 transition.
PP11A-1358
Simulation of the Atlantic meridional overturning circulation at the Last Glacial Maximum
The ocean circulation in the Atlantic deep ocean is characterized by thermohaline circulation driven by deep convection in northern high latitudes. The heat transport associated with this circulation is comparable to that by atmosphere and has a great role in the present climate. The Atlantic meridional overturning circulation (AMOC) is believed to change in past and future climate changes. Coupled model simulations suggest that the AMOC becomes weak in the future global warming climate. Geological evidence such as carbon isotope ratio indicates that the AMOC was weaker and shallower than the present at the Last Glacial Maximum (LGM). As for global warming climate, almost all model results reach consensus that the Atlantic deep circulation weakens in global warming climate. On the other hand, there is wide discrepancy in simulation of the Atlantic deep circulation at the LGM. Weber et al. (2007) report results of Paleoclimate Modeling Intercomparison Project where half of models reproduce the weakening of the Atlantic deep circulation but the other half simulates the strengthening. The reason for this disagreement between models has not been clarified yet, and investigation on the mechanism of weakening of the Atlantic deep circulation at the LGM is one of the most important topics in the paleoclimate studies. In this study, by using results of a coupled climate model (MIROC), we focus on role of changes in the sea surface heat and freshwater fluxes and investigate their role in controlling the AMOC at the LGM. In order to individually evaluate role of heat and freshwater fluxes, we conduct additional ocean general circulation model simulations under the sea surface heat/freshwater flux conditions obtained from the present and LGM simulations by MIROC. The results suggest that the freshwater flux changes contribute to weakening of the AMOC at the LGM, whereas the heat flux changes make the AMOC at the LGM stronger than the present. In the presentation, we are scheduled to report how changes in the heat and freshwater fluxes affect the AMOC at the LGM, and discuss why climate models have difficulty in reproducing the AMOC at LGM, and give implications for mechanisms controlling the AMOC at the LGM.
PP11A-1359
The Variability of the Atlantic Meridional Overturning Circulation in the Last Millennium and two IPCC Scenarios
This work focuses on the simulation and description of the Atlantic meridional overturning circulation (AMOC) variability during the last millennium and its projection into future scenarios. Five simulations performed with the ECHO-G atmosphere-ocean general circulation model (AOGCM) are used in this study: one 1000-yr long control run (CTRL), two forced simulations of the last millennium (FOR1 and FOR2) and two IPCC scenario simulations (A2 and B2) covering the 21st century. In the forced millennial experiments changes in three forcing factors are considered: solar irradiance, the radiative effect of volcanic aerosols and greenhouse gas concentrations. The long-term mean AMOC streamfunction exhibits a realistic circulation, with values close to estimates. The temporal evolution of the AMOC is analysed by means of the Meridional Overturning Index (MOI), defined as the maximum AMOC streamfunction value. The forced runs evidence a weakening of the MOI beginning in the industrial era and increasing in intensity in the future scenario simulations, accounting for a final decrease of up to 40% with respect to the mean preindustrial value. Both the Fourier and wavelet spectrums of the MOI series show an AMOC behaviour close to that of a red noise process; variance exceeds that of an AR(1) at interannual and interdecadal timescales. In the high-frequency (periods < 10 yr), the AMOC is directly responding to atmosphere dynamics, driven by changes in wind-forced Ekman transport at the ocean surface. Several local overturning indices are used, leading to the identification of three main driving patterns: one associated to the NAO, another to ENSO and the last one linked to a local anticyclonic cell over the North Atlantic. All these results are common to the control and the forced runs. In the low frequency (periods > 10 yr) the AMOC is characterized by basin-scale streamfunction anomalies propagating in time. In this case several differences are found among the simulations, and are mainly located in the deep ocean where the forced runs exhibit higher variability than CTRL. A lag-correlation analysis between the MOI and density anomalies at different depths reveals that variations of the AMOC streamfunction are led by the irruption of positive density anomalies in the sinking regions south of Greenland that act to enhance the deep water formation in the Irminger Sea. The effect of an intermittent Greenland tip jet on the variability of convection in the region is finally discussed.
PP11A-1360
Variability in magnetic grain transported by the marine currents in the mid latitude South Indian Ocean and in the Mozambican channel during the interstadials
Bulk magnetic parameters such as volume low field susceptibility (k), Anhysteretic Remanent magnetization (ARM) and Isothermal Remanent Magnetization (IRM) were measured in cores MD94103 and MD002375. These cores were taken by the french R.V vessel Marion Dufresne at mid south latitude locations in the Indian Ocean, during the PACIMA and ANTAUS cruises conducted in 1994 and 2000, respectively. In this sector of the South Indian ocean, magnetic grains are mainly magnetite, and they are transported by the Antarctic circum current (ACC) from the Crozet-Kerguelen plateau area to the sites of the studied cores. A strong variability in the amount of deposited magnetite characterizes interglacials MIS 5, 7, 9 and 11. ARM divided by k, a classical proxy for magnetite grain size, documents concomitant variations, with smallest magnetite grains associated to lowest levels of magnetite concentration. This suggests that the strength of the ACC has varied though time during these interstadials. Bulk magnetic parameters were also measured in core MD022584, taken in the Mozambican channel during the SWAFT-MOZAVITE cruise of the R.V. Marion- Dufresne in 2002. ARM divided by k signal of core MD022584 resembles that of cores MD94103 and MD002375. It suggests that current strength in the Mozambican channel and in the mid South Indian Ocean were closely connected during these interglacials. However, susceptibility signals differ in the two sectors. Implications for south Indian oceanic circulation will be discussed.
PP11A-1361
Outflow From the Amazon: Late Quaternary Changes in the Dynamics of the Surface Water Currents of the Western Equatorial Atlantic
Stable isotope analyses (δ18O) and Mg/Ca palaeothermometry were performed on 4 different species of planktonic foraminifera in order to assess changes in the upper water hydrography of the equatorial Atlantic Ocean throughout the late Quaternary. The Amazon Fan and Ceara Rise, situated at the mouth of the Amazon River, directly underlie the modern flowpath of the North Brazil Current (NBC), a warm water boundary current which flows northwestwards along the Brazilian continental shelf. As a conduit for the cross-equatorial transport of heat and salinity, the NBC is a key component of the heat budget of the North Atlantic and thus of thermohaline circulation. Increases in the latitudinal thermal gradient, associated with the migration of the Intertropical Convergence Zone (ITCZ), result in a periodic weakening of the northward heat flux by way of a seasonal retroflection of the NBC which turns eastwards to form the North Equatorial Counter Current (NECC). Fifteen ODP cores drilled on the Amazon Fan and Ceara Rise were sampled at depths representative of 5 timeslices, selected for their significance in establishing the extent of palaeoclimatic change during glacial-interglacial cycles (modern; early Holocene; Younger Dryas; Last Glacial Maximum (LGM) and Marine Isotope Stage 3). The resulting spatial and temporal distributions of calculated values of δ18Ow (isotopic composition of ambient seawater) are used to infer variations in surface water currents and demonstrate directional shifts in the dispersal of freshwater outflow from the Amazon River. Sea surface temperature reconstructions reveal progressive climatic amelioration over the last 30,000 years, indicating a temperature increase of ~ 3.2 ± 1.1°C since the LGM. In conjunction with this warming, values of Δδ18O, a proxy for water column stratification, are suggestive of increased vertical mixing in the glacial ocean. Spatial variations in δ18Ow imply an oceanward shift in the river outflow plume during cold climates. This directional change, driven by surface water currents, implies a continuation in the formation of the NECC which would cause the curtailment of cross-equatorial heat and salinity transport through a weaker or non-existent NBC. We suggest that a prolongation in the retroflection of the NBC could have resulted from a mean southward migration of the ITCZ during glacial periods.
PP11A-1362
Searching for Abrupt Circulation Shifts in Marine Isotope Stage 2 and 3
During Marine Isotope Stage 3, DO events were recorded in the Greenland ice cores and North Atlantic Ocean sediment records. Some cold DO stadials have been associated with massive freshwater inputs, termed Heinrich Events. These Heinrich Events are frequently associated with "drop dead" circulation periods in which the production of North Atlantic Deep Water is greatly diminished. DO events are thought to result from a restructuring of the overturning circulation. We explore these proposed changes in Atlantic Ocean circulation by examining changes in seawater density in the Florida Straits. The density is inferred from the δ18O of the benthic foraminifera C. pachyderma and P. ariminensis taken from core-sites on the Florida and Greater Bahamas Bank margins. The flow through the Florida Straits is in near- geostrophic balance. This means that the vertical shear in the current is reflected in a strong density gradient across the Straits. During the Younger Dryas and the Last Glacial Maximum the density gradient was reduced consistent with weaker flow through the Straits at these times. A weakening of the Florida Current would be expected if the large scale Atlantic Meridional Overturning Circulation weakened, as has been proposed based on other studies. The Younger Dyras event manifests itself as well-correlated decreases in δ18O from the cores on the Florida margin, while their counterparts taken from the Bahamas remain relatively stable when adjusted for global ice volume. Here, we will present data extending back 32kyr, focusing on those cores taken from the Florida Margin which can resolve millennial scale changes during Marine Isotope Stage 2 and Late Stage 3. We will examine the relationship between circulation changes, as reflected in Florida Margin density, and the three most recent Heinrich events, as well as the most recent DO events.
PP11A-1363
Clay Mineralogy and Clay Geochemistry of ODP Site 1119 – Paleoclimatic and Paleoceanographic History of the SW Pacific for the last 3.9 Ma
Ocean Drilling Program (ODP) Site 1119, off SE New Zealand, contains a ~500 m thick sedimentary record of intermediate-depth drift deposits (Canterbury Drifts) that preserve paleoceanographic and paleoclimatic events of the SW Pacific over the last 3.9 Ma. This study uses clay mineralogy, clay geochemistry, and grain size (terrigenous fractions) to unlock the paleoclimatic and oceanographic trends from the Canterbury Drift deposits. The proxies demonstrate a clear link between regional climatic and oceanographic controls on sediment deposition (e.g. glacial erosion and ocean current flow) as well as global influences (e.g. Milankovitch cycles). At the Early/Late Pliocene boundary (~3.5 Ma), the clay minerals derived from chemical weathering (mainly smectite) change to clays typical of physical weathering and increased erosion (chlorite, illite, and stilpnomelane), marking the transition into a cooler Late Pliocene environment. Shortly after this transition, a marked coarsening of the mean sortable silt fraction occurs at ~3.16 Ma that implies an increased current velocity most likely associated with the initiation of stronger Sub-Antarctic Mode Water flow. After the Late Pliocene (~3.16 Ma) the mean sortable silt fines up through the core, suggesting an overall decrease in ocean current velocity. Meanwhile, increasing chlorite and clay-sized contents up-core provide evidence for the global long term cooling trend and also suggest a stronger influence of glacial erosion processes in the South Island. Around 500 ky, trace elemental concentrations increase coincidentally with the known transition between the dominant 40 ky (Obliquity) and 100 ky (Eccentricity) orbital cycles, which establishes a relationship between the ODP Site 1119 mid-latitude record and global climate and oceanographic change. The proxy analyses from this mid-latitude location provide new information about the timing of environmental changes that are linked to both regional climatic and oceanographic events as well as to global climate change.
PP11A-1364
Ventilation History of the Tropical Atlantic Thermocline: New Insights From the Sensitivity of Foraminifera to Water Mass Nutrient Concentrations
The sensitivity of certain species of foraminifera to nutrient distributions throughout today's oceans highlights their potential for reconstructing water mass nutrient distributions in the past. Applying these new insights to reconstructed abundances of several key species during the last glacial, we find that thermocline waters throughout the entire tropical Atlantic were better ventilated than today. These findings are in line with independent evidence for stronger intermediate-depth ventilation driven by widespread Glacial North Atlantic Intermediate Water (GNAIW), supporting the validity of our new approach. Our results also suggest that well- ventilated GNAIW penetrated at least as far as 25 degrees South, thereby confining the northernmost glacial limits of poorly ventilated Antarctic Intermediate Water (AAIW) to the southernmost Atlantic. We show that the glacial Atlantic thermocline switched to its modern, more poorly ventilated state (probably indicative of a return of AAIW dominance) in a two-step process: a transient reduction in ventilation during the Bolling/Allerod, with the definitive switch to a regime of poor thermocline ventilation occurring at the close of the Younger Dryas. Furthermore, longer-term reconstructions of past distributions of these several key foraminiferal species suggest that a major and enduring impact of glacial-interglacial cycles on Atlantic hydrography has been this vacillating behaviour in tropical thermocline ventilation.
PP11A-1365
Hydrographic Changes In The Bering Sea Responding To The Rapid Climate Transition In The North Atlantic During The Last Deglaciation
The subarctic Pacific region experienced rapid and drastic changes in oceanographic conditions during the last deglaciation, i.e., abundant coccolithophorid production , pronounced carbonate preservation/dissolution , mid-depth ventilation, and dysaerobic intermediate water. Here, we present biogenic opal, CaCO3, stable isotopes of planktonic and benthic foraminifers during the last 25 kyrs in the Bering Sea. A Piston-core was obtained from the Bowers Ridge in the southern Bering Sea (BOW9A; 54°02'N, 178°41'E; 2391 m). Oxygen isotope records of planktonic foraminifer (Neogloboquadrina pachyderma, sinistral) did not present clear patterns of the Bølling-Allerød and Younger Dryas events. On the other hand, changes in CaCO3 contents showed apparent "Greenland type" variations with pronounced peaks corresponding to the Bølling-Allerød and preboreal warming events. Such rapid CaCO3 preservation events were possibly responding to excursion of carbonate ion content of the deep-sea water. Carbon isotope records of benthic foraminifer (Uvigerina spp.) were basically coincident with Pa/Th ratio of the deep North Atlantic, a proxy of the North Atlantic Deep Water (NADW) formation. The δ13C of benthic foraminifer showed lighter values during the Bølling-Allerød and preboreal warming events, whereas higher values during the Heinrich Event 1 and Younger Dryas cooling events. This suggests that deep water with nutrient-rich and high carbonate-ion flowed into the subarctic Pacific region during the warm periods in response to the NADW formation. Relatively high biological production inferred from biogenic opal contents were observed during the deglacial warm periods perhaps due to the results of nutrient supply via vertical mixing from the ocean interior.
PP11A-1366
The Effect of Ocean Circulation changes on the Atmospheric CO2 Concentration: Glacial- Interglacial Cycles
The carbon cycles in the ocean have great influence on the atmospheric CO2 concentration (pCO2, hereafter) because the deep ocean is a much larger reservoir of CO2 compared to the atmosphere-surface ocean system. CO2 in the surface ocean is transported by physical and biological ways to the deep ocean to make a contrast in carbon concentration between the surface ocean and the deep ocean, resulting in lowering pCO2. The global thermohaline circulation (THC) of ocean would have influence both on the physical and biological transport of CO2. It is very important to assess adequately the effect of changes of the global THC on pCO2. We tackle this issue using an ocean general circulation model (OGCM) coupled with a biogeochemical model. This study is important for the better understanding of natural variability of pCO2 as well as the glacial-interglacial cycles. Concerning this problem, we have investigated the effect of various changes in physical conditions of ocean including the circulation field on pCO2 by various numerical experiments. We assumed five model- ocean circulation fields which are based on reproduction by an atmosphere-ocean coupled general circulation model (MIROC3.2). We found that the effect of the difference in the circulation field on pCO2 was very small in every case. For example, if a last-glacial-maximum physical ocean field reproduced by MIROC3.2 was assumed, pCO2 was lowered by ~30 ppm compared to the interglacial value. Most of the 30 ppm reduction can be explained by higher solubility of CO2 into the ocean due to the glacial lower sea-surface temperature. On the other hand, the effect of change in the circulation was less than several ppm. This would be because the variation in physical CO2 transport to the deep ocean and biological counterpart induced by the circulation change would offset each other. The similar characteristic was observed for each of the circulation fields.
PP11A-1367
Continued Melting of Greenland Ice-Sheet Regulated Northern Hemisphere Climate During the Last Interglacial
The evolution of Northern Hemisphere climate during the Last Interglacial (LIG) (129--118 ka) is significant for the study of future climate changes as it may provide information on the climate system responses and feedbacks to radiative forcing (Jansen et al. 2007). We present here a comparison of foraminiferal records from high latitude deep-sea cores with model simulations over the LIG period. We compare high-resolution benthic oxygen and carbon isotope composition records, Sea Surface Temperature (SST) and Ice-Rafted Detritus (IRD) records from a Southern Ocean core with three North Atlantic cores at different water--depth, and one Norwegian Sea core. Our strategy is to correlate in details high latitude sea surface records from both hemispheres with corresponding ice isotopic records using atmospheric markers for the interhemispheric correlation (e.g. Blunier et al 1998; Landais et al 2003). We observe persistent iceberg melting at the beginning of the LIG which maintained relatively cold and fresh surface-water conditions in the North Atlantic and the Nordic Seas between 129 and 125 ka. Similarly, benthic δ13C data indicate different LIG deep-water ventilation patterns, with North Atlantic Deep Waters sinking shallower during the 129--125 ka interval than during the later climatic optimum. The establishment of peak interglacial conditions in the high northern latitudes and associated strengthening of North Atlantic thermohaline circulation were delayed in consequence. Simulations with the IPSL--CM4 ocean--atmosphere coupled model (Marti et al. 2005) suggest that our results are consistent with the impact of a continued melting of Greenland ice sheet on Northern Hemisphere climate, in response to a particularly high boreal summer insolation.
PP11A-1368
Mg/Ca Paleothermometry in the Central Gulf of Cadiz During Heinrich Events
During the last glacial and deglacial period (16kya-46kya), the Iberian Margin experienced local sea surface temperature fluctuations as a result of melting icebergs from catastrophic calvings of the Laurentide Ice Sheet (Heinrich Events). Using Mg/Ca in species of planktonic foraminifera, we are reconstructing temperature for Heinrich Events one (H1), four (H4) and five (H5) from core MD99 2339 in the Gulf of Cadiz (35.88°N; 7.53°W, 1170m). Locally, all three of these events are quite intense showing O-18 excursions of approximately 1.5 parts per mil. Low Mg/Ca values during the Heinrich Events imply significant local and possibly regional cooling. For example, temperatures derived from Mg/Ca during H1 show a temperature oscillation of approximately 7°C between the start and end of the Heinrich Event. While this is a large temperature oscillation, it is of slightly smaller amplitude than the 11°C swing implied by assemblage data for winters (Voelker et al., 2006). By comparing a suite of temperature proxies in the same region (Mg/Ca, planktonic foraminiferal assemblage data and alkenones), we hope to better constrain the climate fluctuations during these time periods as well as gain insight into secondary environmental effects that complicate interpretation of the proxies strictly as temperature. Mg/Ca variations through an entire Heinrich event imply that temperatures in this subtropical region are quite variable and highly dependent on the amount of subpolar waters advected into the Gulf. Using multiple species of foraminifera, we are now attempting to construct a vertical temperature profile prior to, during and after the episode to get a better indication of the penetration of colder temperatures - although investigations are challenging due to low absolute foraminiferal abundances surrounding Heinrich events. Looking at Cd/Ca, we also have new clues about nutrient changes during these time periods, which we hope will lead to a more complete picture of the local and regional environment during Heinrich Events.
PP11A-1369
A 220-ka History (MIS 19 to 31) of Ice-Sheet Variability, Primary Productivity and Sea Surface Temperature in the North Atlantic.
This multi-proxy study of sediments from IODP site U1314 from Gardar Drift focuses on the interactions between the ice-sheet variability and the abrupt changes in the productivity record during the Middle Pleistocene (MIS 19 to 31) time interval from 0.78 to 1.1 Ma. The location of Site U1314, proximal to the IRD belt, provides a record of Heinrich-type detrital layers tracking ice sheet instability, as well as a high resolution record of NADW and its millennial-scale variability. Changes in NADW formation may have been linked to changes in subpolar North Atlantic surface circulation. Sediments recovered from Site U1314 promise to reveal physical and chemical variations in the global thermohaline "conveyer belt", which is strongly influenced by ice-sheet dynamics. We present a comparison of a downcore planktonic foraminiferal and ice rafted debris (IRD) frequency, Mg/Ca and isotopic record and biogeochemical-based record of the productivity. The present record displays a high amount of variability between cold and warm periods, present in all the markers used. Associated with the glacial/interglacial variability is a temporal transition of planktonic foraminiferal assemblages; Neogloboquadrina pachyderma left coiling dominates during glacial periods and Globigerina bulloides accompanied by N. pachyderma right coiling, Globorotalia inflata and Turborotalita quinqueloba during interglacial times. This succession between cold/warm water species also represents the migrational history of the Atlantic Front (AF) at this location. Many of the samples analyzed contain abundant lithic grains, consisting mainly of quartz, volcanic glass and basalt; these were considered IRD and its highest abundance is linked to maximum values in N. pachyderma left. The proxies presented in this study yield a high-resolution record of changes in water column structure, especially during the terminations. Additional examination of the biological response to these water column structure changes will come from geochemical proxies of productivity (i.e., P/Ti, Ba/Ti). Prior to the deglacial period, the lowest SSTs coincide with the highest measured fluxes for planktonic foraminifera and large pulses of terrigenous sediment discharge. We propose that sharp ice-sheet instability during the terminations cause a deep convective mixing, a decrease in the SST and an enhancement in the productivity.
PP11A-1370
The Miocene Climate at 20 and 14 Ma: A Model Study With the Focus on Freshwater
Gradual cooling of the Cenozoic, including the Miocene epoch, was punctured by many warming and cooling episodes. The early and middle Miocene was the time when some elements of the young southern cryosphere might have become unstable and generated significant freshwater impacts on the ocean circulation. It is highly probable that the ocean and eventually the entire climate system responded to those impacts differently in the cases of different land-ocean geometries. We examine the role of freshwater disturbances and significance of ocean geometry in a series of coupled ocean-atmosphere computer experiments. These experiments suggest that relatively small and short-lived fluctuations in ocean-cryosphere interactions could be responsible, at least partially, for notable Miocene climate excursions that cannot be explained by changes in greenhouse gas concentrations, volcanic activities, and/or changes in orbital configuration. The focus of our study is on the Miocene climate response to possible freshwater inputs into the ocean from the Antarctic cryosphere. To simulate freshwater inputs and test our hypothesis of abrupt climate shifts, the freshwater balance at the ocean surface is disturbed by injecting and removing freshwater around paleo- Antarctica. The individual and combined effects of changes in land-sea and ocean bathymetry changes from 20 to 14 Ma are under intense scrutiny. We conclude that recurrent freshwater impacts inside this interval of Cenozoic history could cause significant climate swings with durations from thousand to tens of thousand years.
PP11A-1371
A Box Model of the Miocene Mediterranean Sea With Emphasis on the Effects of Closure of the Eastern Gateway
The proto-Mediterranean Sea in the Early Miocene had two gateways towards open oceans: the Indo-Pacific Ocean in the east and the Atlantic Ocean in the west. Closure of the connection between the Mediterranean Sea and the Indo-Pacific Ocean in the Middle Miocene is thought to have had important effects on the water properties and circulation of the Mediterranean. To gain insight into the circulation of the Mediterranean Sea during the Miocene and the effect of closure we need to understand the Mediterranean exchange flow with the open oceans besides studying the effect of climate on circulation. We address this by means of so-called box modelling. The simplest model has three boxes representing the Atlantic Ocean, the Mediterranean Sea, and the Indo-Pacific Ocean. The boxes exchange water, heat and salt through surface and deep flow. The deep flow is considered density driven. We analyze the variation of the exchange flows in response to changes in the gateways and in atmospheric forcing. The model results are combined with existing observations. Our main conclusions are: (1) Prior to closure deep flow was most likely west to east in both connections provided the density of the Indian Ocean was relatively low. (2) The pre-closure system was probably not very sensitive to atmospheric forcing. (3) Multiple equilibrium states are possible for the Mediterranean in the case of two gateways. (4) The response of the Mediterranean basin to gradual closure is non-linear. (5) Closure probably resulted in cooling and a rise of salinity of the Mediterranean Sea if evaporation exceeded freshwater input. (6) Closure altered the circulation by changing and reversing the western exchange flow. In addition we tentatively propose a best fit scenario for the history of closure linking the available data and our model results. Based on this scenario we suggest a qualitative evolution for temperature which is consistent with existing data.
PP11A-1372
Holocene Climate and Oceanographic Changes in Skagerrak, Eastern North Atlantic
A 32 m long piston core (MD99-2286) was retrieved at 225 m water depth on the north-eastern edge of the Norwegian Channel in Skagerrak during the 1999-IMAGES V (International Marine Past Climate Study) cruise. It spans the latest part of the Younger Dryas and the entire Holocene period. The core site area is promising for high-resolution studies of changes in the oceanic circulation pattern and climate during the Holocene. Today, the circulation pattern in the area of MD99-2286 creates a sink for sediments because inflow of water from the North Atlantic Current and the Jutland Current mix with outflow-water from the Baltic Current, creating an anti-clockwise gyre. This causes a significant decrease in the current speed and allows sedimentation to occur. The age model for the record is based on 27 radiocarbon dates. Foraminiferal contents have been analysed throughout the core, and stable oxygen and carbon isotopes have been measured for the time interval 12000-8500 cal. yr BP. The analyses show a cold ice-proximal environment during the latest Younger Dryas and earliest Holocene. This was subsequently replaced by a more glacial distal environment, initially affected by outflowing melt water from the Baltic area. Around 10000 cal. yr BP, full interglacial conditions were established, and Skagerrak resembled a fjord with a strong inflow of Atlantic water until the eustatic sea level rose sufficiently to flood the entire North Sea, the English Channel and the Danish straits, and the modern circulation system was developed. A major faunal change occurred at ca. 8500 cal. yr BP, and subsequently relatively stable environmental conditions prevailed in the area, though with a slight change in oceanography after ca. 6400 cal. yr BP and particularly during the last 2000 years, when fluctuations increased considerably.
PP11A-1373
Variability in ISOW Vigor Over the Last Millennium and its Relationship to Climate
Resolving the scale and origins of recent low frequency (decadal-centennial) climate variations, such as the Little Ice Age (LIA), is crucial for predicting how natural variability and anthropogenic forcing will interact to affect future climate evolution. These multi-decadal to centennial variations are often postulated to involve, or even be driven by, changes in the Atlantic Meridional Overturning Circulation (AMOC). Assessing this hypothesis for events such as the LIA and the Atlantic Multidecadal Oscillation (AMO) requires records capable of depicting multidecadal changes in past ocean circulation. Here we use well dated (210Pb and AMS 14C), high sedimentation rate, multi and gravity cores taken on the Gardar Sediment Drift (60.19N, 23.58W, 2081 m) to reconstruct decadal to centennial variability in the properties and vigor of the eastern branch of the Nordic Seas overflows over the past millennium. The Gardar drift accumulates on the eastern flank of the Reykjanes ridge due the supply of sediments provided by the overlying Iceland Scotland Overflow Water (ISOW), an important component of NADW. We reconstruct the bottom water physical and chemical properties of ISOW using the oxygen and carbon isotopes of benthic foraminifera (C. wuellerstorfi), while changes in the vigor of near bottom flow are inferred from size variations in the sediment mean sortable silt. Taken together, the records provide a sub-decadally sampled history of ISOW variability spanning from ~900-2002 AD. In addition, changes in surface hydrography are reconstructed using δ18O of the planktonic foraminifera N. pachyderma (d) and G. inflata. Initial results show significant multi-decadal and centennial variability in near surface temperature and/or salinity throughout the last millennium. Although there is a clear warming/freshening trend in our surface water proxies since 1850, similarly large events have occurred repeatedly throughout the record, with the warmest/freshest period in the record occurring in the seventeenth century. Considering these surface water records in the context of regional climate records we discuss the link between decadal-centennial scale climate change and ISOW variability.
PP11A-1374
The Impact of fresh water flux on the Mediterranean thermohaline circulation: models inter-comparison study
Sapropels -organic matter rich layers- are common in Neogene sediments of the Mediterranean Sea. The formation of these layers is known to occur at precession minima and has been attributed to: (1) increased production of organic-matter and (2) increased preservation due to a decrease in the Thermohaline Circulation (THC) and hence oxygenation of the deep waters. In one hand, we assess the response of the Mediterranean THC to the climate system changes (Precipitation, Evaporation and Runoff), and in the other hand, we test the impact of the horizontal grid resolution on the model results. For this goal, we used ocean general circulation models (POM, MOMA and MED16) to simulate the Mediterranean THC for the present day and a precession minimum situation. POM, MOMA and MED16 have a horizontal grid mesh of 1 x 1 degrees, 1/4 x 1/4 degrees and 1/16 x 1/16 degrees respectively. The models are able to capture reasonably the Mediterranean THC components in agreement with the literature. All the experiments show that the fresh water flux (E-P-R) plays an important role in dense water formation. As the precipitation increases, the Mediterranean THC weakens which appears consistent with the "sapropel mode" of circulation. Low salinity values in the Mediterranean basin are found to be associated with the lower model resolution. The insights already gained from this study will serve as a basis for understanding the effects of climate change on a longer time scale. This work is supported by Utrecht Center of Geosciences (Netherlands); Computing was done at VMSG (Netherlands) and at IDRIS (France).