Isotopic and Geochemical Constraints on Paleoclimate Processes II
Presiding: T van de Flierdt, Lamont-Doherty Earth Observatory; F Marcantonio, Department of Earth and Environmental Sciences, Tulane University
PP22A-01 10:30h
Northern vs Southern Hemispheric Controls on Ocean Circulation from Authigenic Nd isotopes in Atlantic Sediments
The first high resolution Nd isotope record of paleocirculation between MIS 1 and 5c from Cape Basin (South Atlantic) cores RC11-83 and TNO57-21 indicates a strong conveyer during warm intervals MIS 1 and 5 and a weaker one during MIS 4 and 2. During MIS 3 the Nd isotope pattern is variable, mimics Greenland paleotemperature variability, and thus reflects NADW intensity. The Cape Basin record shows short-term conveyor strengthening coincident with the most prominent interstadial Dansgaard-Oeschger Events (8, 12, 14, 17) in the Greenland ice record. Local minima at ~40 and ~62 ka may be associated with Heinrich events 4 and 6. During Termination 1 the pattern is unambiguously linked to Northern Hemisphere climate. For example, Piotrowski et al. (2004, 2005) show that the deglacial pattern follows Greenland temperatures and North Atlantic sea-ice cover (e.g. a stronger conveyor with higher Greenland temperatures and an ice free North Atlantic), and is independent of the Antarctic Cold Reversal. Thus the evidence strongly favors a linkage to Northern Hemisphere climate changes. The observed signal in the Cape Basin of conveyor changes can be extrapolated throughout the Atlantic. A survey of cores from 10N to 40S and a range of water depths shows that the Nd isotopes ratios of the authigenic fractions of core top samples are consistent with modern water values. Additionally, there is a consistent trend toward lesser contributions of NADW in the LGM compared to the Holocene throughout. Evidence from multiple proxies consistently indicates a shallower North Atlantic water mass, and mass balance strongly indicates a smaller total production.
PP22A-02 10:45h
Do Geochemical Proxy Records in Cape Basin Sediments Reflect Changes in Deep Ocean Circulation or Surface Ocean Productivity?
Recent studies have reported dramatic correlations between millennial scale variability in the carbon isotopic composition of benthic foraminifera in Cape Basin sediments and contemporary changes in the Nd isotopic composition of authigenic phases (Piotrowski et al., in press) and the Sr isotopic composition of lithogenic phases (Rutberg et al., in press) of these sediments. Factors invoked to account for these features include variability in the supply of North Atlantic Deep Water to the deep Cape Basin, regional changes in the ocean carbon cycle, and iron fertilization of local biological productivity, where the iron may have been associated with lithogenic material derived from South America. Lithogenic material from South America is characterized by non-radiogenic Sr and radiogenic Nd, and variable supply of this material may have influenced the Cape Basin records. To help sort out the relative influence on these proxies of ocean circulation and biological productivity, we compare within a single core records of Nd isotopes, C isotopes in benthic and planktonic foraminifera, concentrations of alkenones and authigenic U, and Th-normalized fluxes of lithogenic material. We find a strong correlation between paleproductivity proxies (authigenic U and alkenone concentration) and the isotopic records of authigenic Nd and of C in benthic foraminifera. The C isotopic composition of benthic foraminifera has some features correlated with the flux of lithogenic material (source of iron) and some features correlated with the C isotopic composition of planktonic foraminifera (proxy for surface nutrient concentration). We conclude that millennial variability in the supply of lithogenic material from South America had a significant influence on biological productivity in the Cape Basin, and through that on the C isotopic composition of benthic foraminifera and on the Nd isotopic composition of authigenic phases in Cape Basin sediments. Changes in supply of macronutrients seems to have been a factor as well. Increased iron supply may reflect surges in the Patagonian ice sheet, while increased surface nutrients may reflect periods of increased ventilation of deep waters. If true, then this scenario implies that the Nd isotopic composition of deep waters during the last glacial period were influenced by processes that are not found to be important today. Further work is needed to test these ideas.
PP22A-03 11:00h
Basin-wide Millennial Cycles in Arabian Sea Climate Over the Last Glacial
High-frequency Dansgaard-Oeschger (D-O) and Heinrich cycles first discovered in the records of North Atlantic ice and marine sediments have been found to extend beyond the North Atlantic There is ample evidence for these millennial cycles of climate variability in the sediments of the Arabian Sea. We employ uranium-series radionuclide proxies to determine changes in the fluxes of sedimentary components in two cores from the western (W) and northeastern (NE) Arabian Sea in order to investigate fluctuations of export production and wind strength on a large regional scale during the last glacial period. In the NE Arabian Sea off of the Pakistani margin, 230Th-derived detrital (eolian) fluxes are highest during periods consistent with the timing of North Atlantic D-O stadial and H 1-7 events. Authigenic uranium concentrations, which we interpret as a proxy for primary productivity, also show an increase during North Atlantic D-O interstadials. Preliminary results from W Arabian Sea sediments off of the Oman margin corroborate that these millennial cycles in productivity and eolian fluxes are indeed basin-wide events. Authigenic U concentrations in these sediments are, on average, about twice those measured in the NE Arabian Sea, suggesting, qualitatively, an enhancement of primary productivity in the western part of the basin. In contrast, fluxes of eolian material to the Oman margin are, on average, more than 10 times lower than those delivered to the Pakistani margin, even though the patterns of millennial variability are virtually identical. We associate enhanced export production and a decreased eolian input during relatively warmer D-O interstadials with an intensification of southwest monsoonal winds. Similarly, decreased export production is coincident with an increase in eolian fluxes during North Atlantic stadial and H events. These results provide strong evidence for a basin-wide atmospheric teleconnection between Arabian Sea and North Atlantic climate on sub-Milankovitch timescales.
PP22A-04 INVITED 11:15h
Millennial-Scale Antarctic Intermediate Water Variability over the past 340,000 Years as Recorded by Benthic Foraminiferal Δ13C in the Mid-Depth Southwest Pacific
Benthic foraminiferal Δ 13C along a mid-depth core from the Southwest Pacific (MD97-2120, 1210m water depth) exhibits high-amplitude oscillations on orbital to millennial time scales. The variability is consistent with rapid changes in Southern Hemisphere intermediate water ventilation over the past 340,000 years. Close correlation of the benthic Δ 13C oscillations with Southern Hemisphere temperature changes suggests direct control of ocean-climate processes at high southern latitudes on the formation and characteristics of Antarctic Intermediate Water (AAIW). Low Δ 13C values during glacial intervals indicate increased upward mixing of deep waters in response to intensified westerlies. Increased northward Ekman freshwater transport stabilized the upper water column and led to reduced rates of intermediate water production, further contributing to decreased Δ13C. These intervals are punctuated by abrupt positive Δ 13C excursions that occur in pace with millennial-scale warm anomalies in the Southern Hemisphere and the relaxation of southern westerly winds. The transient ventilation events directly correlate with episodes of enhanced meltwater discharge into the northern North Atlantic and concomitant reduction in North Atlantic Deep Water (NADW) formation. Conversely, abrupt reductions in AAIW production during mid-Termination cold reversals in the Southern Hemisphere, as documented by parallel negative Δ 13C and SST excursions in the core, correlate with the deglacial resumption of deep overturning in the North Atlantic. The indicated anti-phasing between AAIW and NADW ventilation suggests a tight coupling of Southern Hemisphere meridional overturning to the thermal bipolar seesaw. This adds a THC component to the seesaw and suggests that Southern Hemisphere water mass conversion is actively involved in interhemispheric climate variability.
PP22A-05 INVITED 11:30h
Radiocarbon variability in the Western-North Atlantic during the last deglaciation
The history of the last deglaciation is complex, and to understand it fully we need to be able to compare different parts of the climate system on a common timescale using tracers that inform us on the rate of changes. High-resolution well-dated records of the deep ocean are difficult to construct because of the problems of bioturbation and variable sedimentation rates. Deep-sea corals are an excellent archive of ocean history since their aragonitic, uranium-rich skeletons are well suited to radiometric U-Th dating. This independent chronometer allows us to calculate the radiocarbon content of the water (D14C) in which the coral grew. Moreover, multiple measurements of 14C within a single coral skeleton create high-resolution D14C-transects of ~100 years. More than 30 samples collected insitu cover the last deglaciation at water depths from 1000-2500m. Comparison with radiocarbon measurements from benthic-planktonic foraminiferal-pairs extends this coverage to the last glacial maximum, and to depths >4500m. We interpret the record in terms of mixing of low and high D14C waters, presumably with a Southern and Northern source respectively. We use the rate of change of D14C to help separate mixing events from insitu decay. The WN Atlantic and atmospheric records exhibit some synchronous changes, for instance reduction of radiocarbon levels at the end of Heinrich 1. The upper part of our record, from 1100-1400m had a constant offset from the atmosphere throughout the deglacial. However, at greater depths (~2000m) we see rapid and large shifts, such as a well defined D14C decrease of >100 per mil at 15.4ka that is not reflected in the atmosphere, or in Northern Hemisphere climate records. Our Atlantic record allows us to investigate the controls on the atmospheric D14C record, namely the interplay between the 14C production rate and its primary sink terms; decay and uptake by the deep ocean.
PP22A-06 11:45h
Sr Isotope Variation in U-Pb Dated Permo-Carboniferous Carbonate Cycles of West Texas
Secular variation of Sr isotopes has an intriguing systematic relationship to supercontinental cycles and important biological and climatic events over the past billion years of Earth history. Because the Sr isotope composition of seawater is homogeneous, secular variations must represent changes in continental weathering sources or the balance between continental and mid-ocean ridge inputs into the ocean. This makes Sr isotopes an important tool for chemostratigraphic correlation, although more work is needed to demonstrate that shallow marine deposits in continental platforms are fully mixed with the global ocean. During the Permo-Carboniferous, large glacioeustatic changes in sea level produced high-frequency shallowing upward cycles. Deep water carbonates were deposited during times of highest sea level, and the tops of the cycles record subaerial exposure and meteoric diagenesis as the seas regressed. These cycles occurred on approximately a 100 ky period, based on U-Pb dating of caliches that formed on the subaerial exposure surfaces. Based on cycle stratigraphy and fusulinid biostratigraphy, cycles have been correlated among three high recovery cores of Wolfcampian (Early Permian) to Desmoinesian (middle Pennsylvanian) age from the Central Basin Platform of the Permian Basin of Texas. We selected samples of deep water facies bioclastic wackestones from the high frequency cycles of two of the cores. Two total replicates from each sampled interval were analyzed to evaluate the robustness of the Sr isotope value for recording contemporaneous seawater. Samples with indistinguishable replicate analyses fall mostly within published curves. Additionally we have included analyses of brachiopods from these cores to further evaluate the reliability of the whole rock data. Our results show a trend in Sr isotopes decreasing from 0.70825 from the Virgilian (latest Pennsylvanian) to a low of 0.70791 in the Wolfcampian. Further work on these cores will allow unprecedented resolution of Sr isotope secular variation in this climatic interval when the world was emerging from an icehouse condition similar to the Pleistocene.