PP31D-0649
Decadal-scale Climate Variability in the Gulf of Mexico During the Past 3000 Years
Understanding 20th century global warming depends in part on documenting natural climate variability beyond the range of instrumental records. However, proxy records of decadal-scale climate variability during the past 3000 years are sparse, especially for low-latitude oceans. Generation of proxy climate records from high accumulation rate sediment cores from the Gulf of Mexico can help evaluate climate variability in the low-latitude North Atlantic region. Here we present paired Mg/Ca and d18O measurements on the planktic foraminifer Globigerinoides ruber (white) from two northern Gulf of Mexico sediment cores that provide decadal-scale records of sea-surface temperature (SST) and d18O of seawater (d18Osw) over the past three thousand years (ka). Mg/Ca SST data exhibit a series of six 0.5 degree C oscillations from ca. 3-1 ka before a 2-3 degree C cooling including the Little Ice Age. These oscillations include warm intervals comparable to mid-20th century levels that may correspond to the Medieval Warm Period ca. 1 ka and the Roman Warm Period ca. 2 ka, but also provide evidence for higher-frequency variability. Time series analysis indicates significant periods in the multi-century band (300-600 years) and the multi-decadal band (60-80 years). Comparison to Sargasso Sea records demonstrates a broadly similar pattern of SST change in the low-latitude North Atlantic Ocean during the past 3 ka. Gulf of Mexico d18Osw, which is at present primarily controlled by Mississippi River input and evaporation- precipitation (E-P), seems to be consistent with some North American continental records. In particular, d18Osw maxima at ca. 2.5, 1.05, and 0.5 ka correlate with previously recognized dry intervals in central North America and the Yucatan peninsula. Gulf of Mexico d18Osw values exhibit significant variability, including strong peaks in the multi-decadal band (60-70 years) that are coherent and in phase with SST, but no change in mean (1.0+/-0.3 per mil) relative to core-top values. Overall, these results support models that suggest the Gulf of Mexico and North America have been strongly influenced by decadal- to century-scale changes in regional atmospheric conditions, including the position and strength of the subtropical high pressure cell.
PP31D-0650
Laurentide Ice Sheet Meltwater and Sea Surface Temperature in the Gulf of Mexico During the Late Pleistocene
During Late Pleistocene glacial terminations Laurentide Ice Sheet (LIS) meltwater enters the Gulf of Mexico (GOM). As of yet, it is unclear whether or not weak warm periods, such as marine isotope stages (MIS) 3, 5a and 5c, were associated with meltwater discharge events. It is equally unknown how the timing of these events, if they existed, relates to millennial scale warm intervals determined from Greenland and Antarctic ice core records. To understand the dynamics of LIS melting we investigate the phasing between meltwater events and sea surface temperature (SST) changes. ODP site 625, drilled at a water depth of ~900 m near De Soto Canyon, provide continuous records of MIS 2-6 sampled at a mean temporal resolution of ~400 years. Results from paired Mg/Ca-SST and δ18O on Globigerinoides ruber (white and pink variety) provide evidence for orbital-to millennial influence on salinity and temperature in the GOM. Across the MIS 6/5 termination SST increases ~1000 years before changes in δ18O seawater, a result similar to the most recent deglaciation. Considering that Holocene G. ruber (pink) δ18O values are -1.5‰, MIS 3 and 5 exhibit negative excursions of 0.5‰ or more with values reaching -2.8‰. The greater than 1‰ excursions, which cannot be attributed solely to temperature, suggest significant salinity changes. The timing of southern routing of LIS meltwater has implications for potential disruption of meridional overturning circulation (MOC) during late Pleistocene orbital- to millennial-scale climate changes. However, the initial forcing to instigate large meltwater perturbations is ambiguous. Determining the phasing of regional SST and LIS meltwater events in relationship to high latitude records will increase understanding of important climate feedbacks and the role of the tropics in climate change.
PP31D-0651
High Resolution, Absolute Dated Terrestrial Climate Record of Temperature and Precipitation From the Eastern US Covering 0-7ka, 116-127ka, and 145-298 ka
Analysis of 4 stalagmites from Buckeye Creek Cave (BCC) in West Virginia provides a high resolution record of temperature and precipitation in the eastern US. Periods of coverage include 0-7ka, 116-127ka, and 145-298 ka. Samples were dated using U/Th dating techniques developed for carbonates (Broecker 1963) and adapted for measurement on mass spectrometer (Edwards et al., 1987). The chronology is constrained by 6-14 dates per sample. Replication is the best method to ensure that observed isotopic changes are due to regional climate and not kinetic fractionation or heterogeneous behavior within the cave environment. When replication is available within the BCC record, there is general agreement in the timing, direction, and magnitude of shifts in δ13C and δ18O. Such agreement supports the interpretation of the isotopic composition of speleothem calcite as a climate signal. The δ18O record can reflect either temperature or precipitation. Since the record contains glacial and interglacial intervals and has a range of 2‰ (~5.5 °C at +0.35‰/°C), it is reasonable to conclude that temperature effects determine the isotopic composition of the samples. However, temperature cannot explain the entire record, as Marine Isotope Stages (MIS) 1 and 5e demonstrate more negative values than during full glacial conditions (MIS 6 & 8). Therefore precipitation must be a contributing factor. Such an interpretation is supported by the δ13C record. During arid periods, rock-water interaction time is increased, leading to a positive shift in δ13C (Denniston et al., 2007). Our record is ~4‰ higher during glacial periods than during MIS 1 and 5e. Broadly speaking, our record tracks insolation. However, one remarkable aspect of this record is the behavior of δ18O at insolation peaks. Our record contains four abrupt negative shifts in δ18O during maxima in local summer insolation greater than 520 W/m2. Temperature change does not provide a compelling explanation for this behavior as it would require ~3°C cooling during a period of maximum summer insolation. Therefore, we interpret these rapid shifts as the onset of enhanced precipitation due to an increase in precipitable moisture associated with warmer temperatures or a change in atmospheric circulation. References: W.S. Broecker, Journal of Geophysical Research 68, 2817-2834 (1963). R.L. Edwards et al., Earth and Planetary Science Letters 81, 175-192 (1987). R. Denniston et al., Quaternary Research 68, 45-52 (2007).
PP31D-0652
Opposite trends in sea-surface salinity and temperature between open-ocean and marginal seas in the tropical Western Pacific during the Holocene
The δ18O profiles of surface sea-waters from the Western Equatorial Pacific (MD982170, MD982176 and MD982181 in Stott et al., 2004) show a common, declining trend (~0.7‰) during the Holocene, reflecting a freshening-up of the tropical Pacific. On the other hand, our newly compiled data show that the sea- waterƒnδ18O in the South China Sea (MD972142, MD972151), Celebes Sea (MD12380) and Banda Sea (MD12388) increased through the Holocene by ~0.6‰. This suggests that in the western Pacific when the open-ocean has become less saline, the tropical marginal seas have become saltier. The latter phenomenon is consistent with the interpretation that the East Asian monsoon has become weaker during the past 9 ka as suggested by the increasing δ{18}O trend in speleothems of Doggae Cave (Wang et al., 2005) and Shanbao Cave in southern China (Dong et al., 2006). Furthermore, the SSTs in the Western equatorial Pacific have decreased by about 0.6°C during the Holocene (Stott et al., 2004) while the SSTs in the NE South China Sea (MD972142) shows a rising trend of ~1.0°C. On the other hand, the SSTs at other sites of the studied tropical marginal seas do not show any distinctive trends.
PP31D-0653
Coastal Lakes in South-Central Chile (38°S) – Archives of Holocene Climate and Fore-arc Tectonics
The present study is based on sediment cores obtained from the two coastal lakes Lago Lanalhue and Lago Lleu Lleu, which are located on the western slopes of the Chilean Coastal Cordillera at approximately 38°S. Due to their near-shore location outside the influence of the Andean glaciation and volcanic activity, the lake sediments provide clear signals of continental rainfall changes that can be directly linked to offshore marine records. Both lakes are of tectonic origin and were cut off from the Pacific Ocean by raised marine terraces forming natural dams. For that reason their sediments do not only contain paleoclimatic information but also information about fore-arc tectonics and, in particular, about the regional uplift history. Sediment analyses so far include the lithological description, digital imaging, magnetic susceptibility measurements, X-ray fluorescence (partly high-resolution) scanning, carbon content detection and radiocarbon dating. Investigated piston cores of both lakes are extending back to the Late Pleistocene/Early Holocene and are showing mean sedimentation rates of 1.0 - 1.5 mm/a. First results of magnetic susceptibility measurements and XRF-scanning demonstrate a striking shift of the respective values at around 8000 yr BP in both lakes, which documents the transition from a marine to a lacustrine facies. Knowing the Holocene sea-level evolution and the present height of lake-levels we are able to calculate the mean regional uplift rate during the last 8000 yr (4.3 mm/yr). In the lacustrine part of the records we find higher terrigenous content during the Late Holocene (4300 to present) compared to the Middle Holocene (7600 – 4300 yr BP) consistent with generally more humid conditions in the late Holocene as known from other terrestrial and marine records in the region. On centennial time-scales a pronounced variability in elemental concentrations and magnetic suceptibility shows significant changes in the terrigenous sediment input to the lake, suggesting important hydrological fluctuations also on these time-scales.
PP31D-0654
Geophysical Evidence for Holocene Lake-Level Change in Southern California (Dry Lake): Additional Evidence for a Regional Early Holocene High Stand
Ground penetrating radar data are used to develop a Holocene history of basin sedimentation in a small, alpine lake in Southern California (Dry Lake). We define three depositional sequences spanning the past 9,000 calendar years before present (cy BP). Although, the basin contains sediments clearly older than the Holocene, we focus on the past 9,000 cy BP to match our similarly aged sediment cores. Sequence I represents the first phase of an early Holocene high stand. A major regression, perhaps following the 8,200 year cold event, separates Sequence I from Sequence II. The timing of this regression is approximately coeval with major regressions at Owens Lake (Bacon et al., 2006) in southeastern California and Tulare Lake (Negrini et al., 2006) in the southern Central Valley of California. Sequence II represents the second phase of the early Holocene high stand. This second high stand phase is also observed at Tulare Lake but not at Owens Lake. Sequence III represents a permanent shift to low or dry lake stands. By 4,000 cy BP, the lake earns rightfully its appellation of Dry Lake as indicated by a permanently contracted central basin. The similarity in ages of early Holocene lake level change across the greater region of Southern California suggests a similar external forcing – perhaps modulation of early Holocene storm activity by insolation (Kirby et al., 2007). The lake level records are less congruous for the mid-to-late Holocene across the region. Specifically for Dry Lake, it is not clear what caused the apparently rapid shift from a deep, early Holocene lake to a permanent shallow or dry lake by the mid-Holocene.
PP31D-0655
Validation and use of General Circulation Models (GCMs) for past and future hydrological studies
Comparison of numerical model simulations for the Late Cretaceous, present day, and the future suggest both an enhanced hydrological cycle and a fundamental change in the relation between surface and subsurface runoff during past and future greenhouse times. Nine climate simulations have been run for the Late Cretaceous (5 models using 6 times modern atmospheric CO2 and four different orbital configurations representing one full precessional cycle), the present (1 model), and the future (3 models using modern geography; (1) 6 times modern atmospheric CO2 (2) plus Cretaceous soil composition, and (3) plus Cretaceous vegetation. The paleoclimate simulations of the Cretaceous suggest that on a global scale total river discharge was increased by ~34%, surface runoff was reduced by ~33%, and subsurface runoff was enhanced by ~60% compared to today. Similar proportions have been simulated for the future if CO2 continues to rise to Late Cretaceous values (i.e. 6 times modern values) using soil composition and vegetation as for the Late Cretaceous. To validate these past and future models we compare the results from the present day model run with instrumental data from hydrographic measurements. We observe strinking similarities between modelled and measured data both on a global and regional scale supporting the conclusion that current GCM do well represent natural conditions. As suggested by the geological record, these findings emphazise the importance of changes in the hydrological cycle at different scales as they enhance deep chemical weathering in particular under tropical conditions. As a result these processes are expected to result in enhanced continental nutrient export to the coastal ocean, strongly affecting ocean chemistry (O2 CO2, C, and nutrient cycling) and impacting on future climate change. This study once again highlights the crucial role of terrestrial-marine interactions both for past and future climate change.
PP31D-0656
Hydrological forcing and circulation responses in a miniature ocean
The Mediterranean Sea is often referred to as a miniature ocean as it has many processes in common with the world ocean with respect to its circulation, albeit at smaller temporal and spatial scales. Specifically, the basin s freshwater and heat budgets, tied to the high vs. low latitude climate interactions over the region, drive an efficient thermohaline circulation (THC). Variations in the freshwater budget are especially dominated by processes that act in the eastern Mediterranean. During periods of precession minima, enhanced river discharge along North African margin due to intensified African summer monsoon circulation weakens the eMed THC, leading to widespread seafloor anoxia (sapropels). The eMed thus represents an ideal target to test the sensitivity of a THC system to changes in the net freshwater input. At the same time, the anoxic sedimentation allows high-resolution study of the basin s responses to high latitude climate variability during periods of monsoon maxima. Here we present highly resolved Aegean (NE eMed) records of foraminiferal stable isotopes, census counts, dinoflagellate cysts, and organic biomarkers through two key sapropels from the last (S5) and the current (S1) interglacial periods. During the last interglacial period, a severe monsoon-fuelled freshwater discharge into the open eMed promoted a virtually immediate (within 40 yr) collapse of the Aegean deep overturning system, a key source of eMed deepwater. This THC shut down was accompanied by a burial of exceptionally high amounts of organic carbon (up to 14% Corg) in the SE Aegean site of core LC21. These changes were followed (within few centuries) by the expansion of euxinic conditions toward the photic layer first in the Aegean and subsequently throughout the eMed. During the early to mid Holocene interval of S1 deposition, the monsoon-fuelled freshwater discharge into the eMed was less extreme than its S5 counterpart, leading to a somewhat muted response of the eMed THC and less enhanced carbon burial (up to 2% Corg). Superimposed, we identify two sharp episodes of intense Aegean winter cooling between ~9.8 and ~7.9 ka BP in central Aegean Sea core SL21. These cold spells are found to match changes in Greenland ice cores, pointing to meridional displacements of the atmospheric polar vortex as a connecting mechanism. The timing of these two cold events is discussed within the context of recently reconstructed re-ventilation events punctuating S1. Combined with the instrumental records of recent changes in the basin s THC, our findings hint at an exceptional sensitivity of the eMed to decadal- to millennial-scale hydrological perturbations in the tropics/subtropics. Changes in both the high and low latitude climates are found to profoundly affect heat, freshwater, and carbon budgets of this miniature ocean.
PP31D-0657
Late Miocene Freshwater Runoff Seasonality Inferred by LA-ICP-MS and TIMS Analyses on Eastern Mediterranean Corals
Distinct trace element ratios in corals (e.g., Sr/Ca, U/Ca, Ba/Ca) can be used to monitor environmental conditions such as sea surface temperature (SST), soil erosion, or river runoff. Only trace element ratios from diagenetically unaltered corals yield results suitable for environmental reconstructions. Generally, such material is only available from strata of Pliocene or younger age. However, we found exceptionally well-preserved Porites corals of Late Miocene age on Crete (Eastern Mediterranean) in a succession of various clastic sediments. Previous studies on stable isotope (δ18O, δ13C) compositions yielded SST seasonality patterns, similar to those found in present-day corals. In this study, trace element concentrations in coral skeletons of these samples were measured using a 213 nm Nd:YAG laser coupled to an Element2 LA-ICP-MS (ThermoFisher) at a spacial resolution of 500 μm along the axis of maximum growth, corresponding to eight to eleven samples per year. Also, two 87Sr/86Sr profiles were measured using a multi-collector TIMS (ThermoFisher Triton) on powder samples gained by equidistant microdrilling (spherical 0.8 mm bit) at a resolution of 4 samples per year. Both 87Sr/86Sr profiles run parallel to the sampling transects of the LA-ICP-MS and previously published stable isotope analyses representing a time span of 7 years. Sr/Ca and U/Ca co-vary with δ18O, and thus closely reflect SST seasonality. Other element ratios (e.g. Al/Ca, Ba/Ca, REE/Ca) do not correlate with δ18O (and thus SST), but do, however, exhibit well- defined peaks in the winter months. This pattern is not considered to originate from diagenetic alteration or post- growth contamination. Rather, these element ratios most likely represent seasonal terrigenous input into the coral reef environment syn-genetically incorporated into skeletal porosity as non-lattice bound components during coral growth. This pattern could arise from seasonality in Saharan dust plumes. However, because of its winter dominance it more likely reflects variations in riverine suspended load from Crete into the near-shore environment of the corals caused by winter rainfall. This is in good agreement with our SEM results indicating terrigenous particles trapped and incorporated into skeletal parts grown during the winter. In this case, the non- lattice bound element ratios are proxies for the frequency and intensity of heavy winter rain events and associated plumes of local turbid fresh water in the Eastern Mediterranean during the Late Miocene. The 87Sr/86Sr ratios cluster closely around 0.708910, confirming an age of ~9 Ma for the corals, and suggest a working hydrographic working of the Eastern Mediterranean Basin to the open ocean at that time. Nevertheless, several Sr isotope excursions (down to 0.708676) occur in the summer seasons, requiring additional input of unradiogenic Sr. These excursions do not correspond straightforwardly to the trace element patterns, implying that 87Sr/86Sr variability is unrelated to winter sediment discharge events. Although the Nile has suitably unradiogenic 87Sr/86Sr, its low dissolved Sr concentration effectively rules it out as a viable source. At present, the origin of the Sr isotope excursions remains enigmatic.
PP31D-0658
Sensitivity of Ocean Circulation and Tracer Distributions to Changes in Surface Heat and Freshwater Fluxes in an OGCM
We vary the sea-surface restoring temperature and salinity in an ocean general circulation model (OGCM) to obtain a suite of unique ocean circulation states. We find that the differences in implied surface heat and freshwater fluxes between model runs are relatively small (within the error of previous estimates for the modern ocean), but observe significant differences between runs in the large-scale ocean circulation. In particular, the relative fractions of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) in the ocean interior vary widely between runs. The fraction of the interior ocean ventilated from the North Atlantic is observed to vary between 12% and 61% throughout the entire range of sensitivity runs, while the corresponding fraction ventilated from the Southern Ocean varies between 68% and 15%. The sensitivity of northern and southern end-member water mass fractions to small changes in surface heat and feshwater fluxes has important implications for interpreting paleoceanographic records. Records of past ocean tracer concentrations are an important source of information in reconstructing the past ocean circulation. For example, it has been argued that increases in the top-to-bottom radiocarbon age in the North Atlantic at the Last Glacial Maximum (LGM) resulted from a weaker North Atlantic overturning circulation, in turn supporting reduced northward heat transport. In contrast, we show that top-to-bottom radiocarbon ages in the North Atlantic can increase by 400-500 years with little change in the strength of the overturning circulation or in the ocean northward heat transport. This is achieved without changing the air-sea equilibration timescale for radiocarbon. Our results highlight the difficulty of estimating past ocean transports from paleoceanographic tracer data. Furthermore, if the real ocean is as sensitive to changes in surface heat and freshwater fluxes as our model suggests, we could expect significant temporal variability in tracer concentrations without drastic changes in the energy or hydrological cycles.