PP12A-01 INVITED
Rain driven by receding ice sheets as a cause of past climate change
The Yonger Dryas cold period is one of the most dramatic incidents of abrupt climate change reconstructed from paleoclimate proxy records. Changes in the Atlantic Ocean overturning circulation in response to freshwater fluxes from melting ice are frequently invoked to explain this and other past climate changes. Here we propose an alternative mechanism which is supported by the results of a state-of-the-art global climate model. We show that reducing the size of the Laurentide Ice Sheet can cause the model atmospheric circulation to enter a regime characterized by greater net precipitation in the North Atlantic region. This leads to a significant reduction in ocean overturning circulation, causing an increase in sea ice extent and hence colder temperatures. Positive feedbacks associated with the sea ice amplify the cooling. This suggests that the atmospheric precipitation response to receding glacial ice sheets could have contributed to the Younger Dryas cooling, as well as other past climate changes involving ocean overturning circulation.
PP12A-02
Meridional reorganizations of marine and terrestrial productivity during Heinrich events
To study the response of the global carbon cycle to a weakening of the Atlantic Meridional Overturning Circulation (AMOC) a series of freshwater perturbation experiments is conducted both, under pre-industrial and glacial conditions using the earth system model of intermediate complexity LOVECLIM. A shutdown of the AMOC leads to substantial cooling of the North Atlantic, a weak warming of the southern hemisphere, intensification of the northeasterly trade winds and a southward shift of the Intertropical Convergence Zone (ITCZ). Trade wind anomalies change upwelling in the tropical oceans and hence marine productivity. Furthermore, hydrological changes associated with a southward displacement of the ITCZ, lead to a reduction of terrestrial carbon stocks mainly in northern Africa and northern South America, in agreement with paleo-proxy data. In the freshwater perturbation experiments, the ocean acts as a sink of CO2, primarily through increased solubility. The net atmospheric CO2 anomaly induced by a shutdown of the AMOC amounts to about +15 ppmv and --10 ppmv for pre-industrial and glacial conditions, respectively. This background state dependence can be explained by the fact that the glacial climate is drier and the terrestrial vegetation therefore releases a smaller amount of carbon to the atmosphere. This study demonstrates that the net CO2 response to large-scale ocean circulation changes has significant contributions both, from the terrestrial and marine carbon cycle.
PP12A-03
A new Reconstruction for the Onset of the Younger Dryas: Evidence From Speleothem, Tree Ring, and Marine Varve 14C Archives and Greenland Ice Core Proxies
Attempts to establish the causes and timing of the Younger Dryas (YD) cold event have been seriously hindered by major discrepancies in varve, ice core, and radiocarbon chronologies. Here we compare new 14C results from a 230Th-dated Hulu Cave speleothem from eastern China (Wang et al, 2001) with marine 14C data from varved Cariaco Basin sediments from the tropical North Atlantic (Hughen et al, 2000), and use the results to develop a new reconstruction for the YD onset. We see no evidence for hypothesized increases in ocean-atmosphere radiocarbon offsets (reservoir ages) in the deglacial North Atlantic, proposed on the basis of the 14C record from a floating Allerod tree ring sequence (Kromer et al, 2004). Instead, the data imply that the dendro record should be shifted from its original placement to younger calendar ages by almost 300 years. This brings it into good agreement with YD chronologies in European lake varves, but seriously violates an assumed hemispheric synchronism for the Allerod/YD transition, since proxies from Cariaco, Hulu, and the GISP2 ice core all show major climate shifts beginning ~12,900-13,000 BP, some 200-300 years before the onset of the European YD. In this context we interpret a period of remarkably low reservoir ages in the Cariaco data as recording a brief but total shutdown of the Atlantic Meridional Overturning Circulation that began around 12,950 BP and led to intense stratification and near-equilibration of the upper waters of the North Atlantic subtropical gyre with atmospheric 14C. A Little Ice Age (LIA) centennial-scale cooling event of solar origin, recorded by an intense peak in cosmogenic 10Be concentrations in GISP2, cooled the central North Atlantic sufficiently to restart overturning ~12,700 BP, but in a southward-shifted mode that left much of Europe north of the polar front. This LIA cooling, and not the AMOC shutdown that preceded it, marks the start of the European YD.
PP12A-04
Last Deglaciation Events (16.1-11.4 cal-Ka) Recorded in a Speleothem from DeSoto Caverns, Alabama, U.S.A.
Whereas the rapid climate swings that occurred during the last deglaciation have been well documented in the Greenland ice cores, their cause/s continue to be a subject of heated debate. Clearly, more geographically dispersed records are required in order to provide better insight into the history of deglaciation, and by extension into the cause/s of the abrupt climate shifts. Particularly scarce are continental deglaciation records from the southeast North America whose atmospheric conditions were controlled by the retreating Laurentide Ice Sheet to the north and the Gulf of Mexico warm waters to the south. In order to remedy the absence of deglaciation records in the Southeast USA in general, and the Gulf Coast in particular, we have initiated a study of a 55-cm long stalagmite (DSSG-2) from the DeSoto Caverns in Childersburg, Alabama (33° 18'N, 86° 17'W). Seven radiocarbon AMS and eighteen U/Th TIMS dates reveal that the continuously layered stalagmite covers the time interval 31 to 11.4 cal-Ka at growth rates varying from 61 μm/decade at the start of deglaciation and up to 2700 μm/decade close to its termination. The combination of unusually high growth rates, pristine aragonite mineralogy and tight sampling (n=602) afforded generation of high fidelity δ13C and δ18O records from about 16.1 to 11.4 cal-Ka whose high resolution is comparable with the contemporaneous Greenland ice core records. The stalagmite δ18O record shows excellent agreement in relative amplitude shifts and timing of abrupt and brief cold reversals (Oldest Dryas, Older Dryas, Inter-Allerød Cold Period) that punctuated the overall trend of deglaciation warming (Bølling/Allerød period). The succeeding Younger Dryas is depicted in the stalagmite by rapid positive shifts in δ18O and δ13C of 1.3‰ and 2.3‰ (V-PDB) relative to the baseline mean value and its start and termination (12.7-11.8 Ka) are concordant within error with the dates reported from GISP2 ice core (12.82-11.60 Ka). We interpret the negative isotope shifts during the warm B/A interval and the positive shifts during the cold ODs and YD intervals as being primarily an expression of rapid shifts in rainfall amount alternating between excessive floods and severe droughts. The excellent correspondence observed between the DeSoto stalagmite and the GISP2 isotope records further suggests that disturbances in thermohaline circulation caused by repeated fresh water discharge episodes into the North Atlantic from the retreating American and European ice sheets were the principal governing factors controlling the documented isotope shifts in the Southeastern USA and Greenland.
PP12A-05
Holocene Southern-Ocean Surface Radiocarbon Ages: Implications for Ocean Circulation and Ice-shelf Flow Rates
The Southern Ocean features high surface-water 14C reservoir ages, reflecting substantial upwelling of old deep water and poor air-sea exchange. These high values complicate 14C dating in the circum-Antarctic region, and encode information about past ocean circulation (particularly the rate of deep-water ventilation in the Atlantic). Here we present new results from the Ross Sea that provide a history of Holocene Southern Ocean 14C. Freezing at the base of the McMurdo ice shelf traps sediment (including solitary corals) which is then transported to the ice surface by ice ablation. Forty-five solitary corals from the McMurdo Ice Shelf and from Hells Gate have been dated precisely using U/Th and 14C techniques to provide a detailed reconstruction of surface-water reservoir ages for the past 6,000 years. With the exception of two young samples that show the impact of bomb radiocarbon, other samples indicate a constant 14C reservoir age during this period of 1300±200 years. The constancy of this value is reassuring for studies conducting chronology in the Southern Ocean, or relying on knowledge of deep-water source regions for 14C ventilation ages. It also allows constraints to be placed on changes in the flow of NADW with time, since slower flow leads to older upwelling water in the Southern Ocean. The systematic increase in age of samples with distance from Black Island also allows reconstruction of the flow rate of the McMurdo Ice Shelf. This indicates a constant flow of about 4 m/yr for the last 5000 years, with flow about three times faster before this period. These flow rates compare with short term estimates of flow in the region of about 16 m/yr suggesting that flow may have increased in recent times.
PP12A-06
Holocene Instabilities in GIN Sea Overflows Based on Mineralogical and Nd/Pb Isotopic Sedimentary Records
Grain size measurements, clay mineral assemblages, Nd and Pb isotope compositions of fine fractions from core MD99-2322 from the Irminger Basin (67°N, 305°W, 714 mbsf) are used to document relative contributions from the proximal Denmark Strait (Denmark Strait Overflow Water, DSOW) and distal overflows from the Iceland-Faroe-Scotland sill (North East Atlantic Deep Water, NEADW) to deep current transport during the Holocene. Sortable silts indicate instability in current velocity, especially between 7000 and 2500 yr. cal. BP. Since 7000 yr. cal. BP., lower smectite/illite ratio records higher illite-rich supplies, probably from proximal Greenland. On an isotopic Pb and Sm/Nd mixing diagram, i) Mid-Atlantic Ridge basalts (MAR), ii) Greenland Panafrican Crust (GPC) and iii) European Panafrican Crust (EPC) sources represent the major end-members. Most Nd-Pb signatures are explained by a mixing of the three end-members. According to previous investigation, GPC constitutes the isotopic fingerprint of DSOW, EPC of NEADW. From 7000 years BP onwards, we note an increase of the GPC attributed to a gradual contribution of the DSOW within the deep circulation. A few "isotopic excursions"" occur in addition. Those pulses are made by a sharp increase of GPC contribution (70 %), with no EPC contribution. These pulses are observed in phase Bond's Holocene ice-export events in the NE Atlantic: they indicate a total collapse of NEAW export. During the last 2500 yr cal. BP, relative GPC contribution reaches 40 %. It can be concluded that since 2500 yr cal BP DSOW represents the major deep water source within NADW. Therefore establishment of modern oceanic deep circulation occurs during the Late Holocene. Our data highlight a close relation between Holocene climate variability and deep oceanic circulation.
PP12A-07
Indonesian Throughflow dynamics of the last 25 kyr: New Uranium Series results from the Indonesian Seas
In the modern ocean the Indonesian Throughflow (ITF) is a key component of thermohaline circulation (THC) that transports large amounts of heat from the Pacific to the Indian Ocean and is the only tropical connection between major ocean basins. This oceanographic system is known to influence a number of climatic conditions, one significant example being El Niño Southern Oscillation (ENSO). Undoubtedly, the Indonesian seas are important for climate, yet we have only a modest understanding of how this region operated in the past. During the Last Glacial Maximum (LGM) the Indonesian Seas experienced significant sea level regression with consequent changes in boundary conditions (De Deckker et al., 2002). Previous paleo-studies within the Indonesian Seas implied a weakening of the ITF during the LGM (Müller and Opdyke, 2000; Gingele et al., 2001). These interpretations focus on paleoproductivity and sedimentology proxies rather than proxies that more directly measure paleocirculation. In addition, previous interpretations of paleoproductivity may require clarification due to possible post depositional sediment focusing in this region. We present two down core records from the Timor (MD-982167) and Flores Seas (VM33-80) that span back ~25 kyr. We use 231Pa/230Th as a more direct paleocirculation tracer and apply 230Th normalized fluxes and biogenic opal in order to look at paleoproductivity during the LGM, through the deglaciation and into the Holocene. These records extend on current paleooceanographic knowledge from the equatorial realm of the climate system. More specifically the records aim to better understand an important segment of the THC, namely the ITF, and how its strength has interacted with climates of the past. De Deckker P., Tapper N. J., and van der Kaars S. (2002) The status of the Indo-Pacific Warm Pool and adjacent land at the Last Glacial Maximum Global and Planetary Change 35, 25-35. Gingele F. X., De Deckker P., and Hillenbrand C. D. (2001) Clay mineral distribution in surface sediments between Indonesia and NW Australia - source and transport by ocean currents. Marine Geology 179, 135-146. Müller A. and Opdyke B. N. (2000) Glacial-interglacial changes in nutrient utilization and paleoproductivity in the Indonesian Throughflow sensitive Timor Trough, eastmost Indian Ocean. Paleoceanography 15(1), 85-94.
PP12A-08
The Adjustment of Ocean Tracers at Millennial Timescales
Interpretation of climate change as recorded in marine sediment cores requires an understanding of the advection, mixing, and (in some cases) decay of oceanic tracers. Even a steady-state ocean make take many millennia to come into full equilibrium with a surface anomaly, depending on the location of the anomaly and type of tracer. The problem is general and in some ways much studied, but here we introduce two novel aspects. One, we use a global ocean circulation model forced into consistency with a large collection of modern-day observations, as developed by the ECCO Consortium, to provide our most realistic guess at the circulation. Two, we attempt to provide further insight through an intercomparison of various measures of ocean timescales. In particular, we have conducted a very long simulation of the adjustment of conservative and radioactive tracers to surface anomalies over tens of thousands of years, and compare measures of the timescale of these adjustments as derived from radiocarbon activity, lead-lag relationships, equilibration timescales, and volume-filling rates. Furthermore, we demonstrate a Green function technique to reconstruct the transient response of a suite of tracers at any point in the ocean to arbitrary surface anomalies over many millennia. As a specific example, we consider how observations of benthic proxies lagging their planktic counterparts through the last deglaciation may be understood in the context of ocean adjustment timescales.