PP31B-01 INVITED 08:00h
Evidence For Deep-water Production In The North Pacific During The Early Cenozoic
The evolution of Cenozoic climate began with a long-term warming trend culminating in peak warmth during the Early Eocene. This was followed by intense cooling that ultimately led to the onset of the "icehouse" climate of the late Cenozoic. Changes in poleward heat transport via thermohaline circulation may have played a role in the general evolution of Cenozoic climate. To investigate the relationship between the evolution of global climate and thermohaline circulation over the interval $\sim$70 to 30 million years ago (Ma), I generated paleo-seawater neodymium isotopic records from Ocean Drilling Program (ODP) sites in the present-day northern Pacific Ocean. Fish debris Nd isotope data from ODP Sites 1209 and 1211 (paleodepths $\sim$2300m and $\sim$2900m) indicate that $\epsilon$$_{Nd}$ values in the deep waters of the central subtropical Pacific were characterized by a composition of $\sim$-4.5 to -5 during the latest Cretaceous and the Early Paleocene ($\sim$70 to 64 Ma). Beginning $\sim$64 Ma, the composition of Pacific deep waters became more radiogenic ($\sim$-3 to -3.5), and remained so for the next $\sim$20 million years. From $\sim$46 to 33 Ma, deep-water $\epsilon$$_{Nd}$ values subsequently became more nonradiogenic ($\sim$-4.5 to 5). The similarity in the overall $\epsilon$$_{Nd}$ trends at both sites indicates that they were bathed by a common deep-water mass. Nd isotopic data from ODP Leg 199 Sites (1215, 1219, and 1221; north-central Pacific), which lie $\sim$6000 km to the west and south of Shatsky Rise, demonstrate a trend similar to the upper portion of the records from Shatsky Rise. Although the basal age of sediments from the Leg 199 Sites is younger than 56 Ma, $\epsilon$$_{Nd}$ values from Sites 1215, 1219 and 1221 demonstrate the shift from relatively radiogenic (North Pacific) values of -3.5 at 53.7 Ma to more non-radiogenic (Southern Ocean) values of -4.9 by 44.8 Ma. The $\sim$1.5 epsilon unit shifts toward more radiogenic values recorded at both sites reflect a fundamental change in the source of deep waters bathing the northern Pacific sites from $\sim$65 to 45 Ma, involving a switch in deep-water production from the Southern Ocean to the North Pacific. Production of North Pacific deep waters lasted $\sim$20 million years, and coincided with the warmest climatic interval of the Cenozoic Era. The timing of the deep-ocean circulation changes lagged changes in global temperatures implying that climate exerts long-term control over thermohaline circulation patterns. Thus Cenozoic thermohaline circulation patterns evolved in response to long-term changes global climate, and were not the cause of extreme Cenozoic warmth.
PP31B-02 INVITED 08:15h
Tracking Warm Saline Deep Water on Maud Rise Using Nd Isotopes
Evidence for the production and export of Warm Saline Deep Water (WSDW) to the southern high latitudes during the Paleogene is equivocal and widely debated. The strongest support for WSDW in the Southern Ocean are discrete intervals of benthic $\delta$$^1$$^8$O inversions between vertically offset sites on the Maud Rise, ODP sites 689 (2080m) and 690 (2914m), which were interpreted as temperature inversions. During these intervals a warm bottom water mass is believed to have occupied site 690. Nd isotopes in fossil fish teeth reflect water mass mixing and are unaffected by changes in temperature, salinity, nutrients, or productivity. We present Nd isotope records for sites 689 and 690 that cover the middle Eocene to the late Oligocene and which have been correlated by Sr isotope chemostratigraphy. The Nd isotope data support the incursion of a distinct water mass at depth in the middle Eocene and portions of the Oligocene. During these intervals $\epsilon$$_N$$_d$ values at site 690 are 1$\epsilon$ unit less radiogenic than those at site 689, reaching minimum values of -9.3 to -10. The $\epsilon$$_N$$_d$ minima at site 690 coincide with $\delta$$^1$$^8$O inversions. In the Paleogene, potential sources of deep water masses characterized by such nonradiogenic $\epsilon$$_N$$_d$ values include the North Atlantic, Southern Ocean, and Tethys Sea. However, the North Atlantic is an unlikely source of such a strong signature to the high latitude Southern Ocean and Paleogene $\epsilon$$_N$$_d$ values of Southern Ocean deep waters are too radiogenic to account for the Maud Rise Nd data. Further, we argue that a Southern Ocean water mass should influence both sites, and the data clearly support instances when more than one water mass was present. The Maud Rise Nd isotope data is best explained by the export of Tethys sourced WSDW to the Southern Ocean. Authigenic shelf deposits from obducted Tethys margin sediments indicate that shallow Tethys seawater had $\epsilon$$_N$$_d$ values of -9.2 to -9.7, suggesting that deeper waters were less radiogenic. These values are also consistent with modern Mediterranean outflow (-9.4). The pulsed export of WSDW to the Southern Ocean from a low latitude source accounts for the coincidence of $\epsilon$$_N$$_d$ minima at site 690 with $\delta$$^1$$^8$O inversions between sites 689 and 690.
PP31B-03 INVITED 08:30h
A High Resolution Record of Ocean Circulation since the last Interglacial from Nd isotopes
The variability of the global conveyor through ice ages has been a matter of dispute due to different indications from different paleocirculation proxies. Nd isotopes in marine precipitates conservatively reflect water mass mixing along their transport paths, and are changed only by addition of a new component of Nd. We present the first high resolution Nd isotope record of paleocirculation between MIS 1 and 5c from Cape Basin cores RC11-83 and TNO57-21. The record indicates a strong conveyer during warm northern hemisphere interglacial and interstadial intervals, and a weaker one during glacials and stadial intervals. The pattern of changes varies with Greenland paleotemperature. A first quantification attempt indicates that NADW export during the LGM was half the magnitude of the early Holocene. The strengthening through Termination 1 covaries with Greenland temperature and North Atlantic sea ice coverage. It markedly weakens between the Bolling and Younger Dryas. The weakening during the MIS 5a/4 transition is sharp, occurring within 1 ka. Throughout MIS 3 short-term maxima are associated with prominent interstadials in Greenland ice records (8, 12, 14, 17). Local minima at ~40 and ~62 ka may be associated with Heinrich events 4 and 6. During the ice age initiation and termination, global carbon budget shifts preceded circulation changes, and these were preceded by ice sheet growth during the initiation. There is no apparent lead-lag during interstadial warmings. The observations allow for ocean circulation to be a trigger of major climate change during abrupt events that are not driven by orbital forcing. At major glacial-interglacial boundaries, the global carbon budget and thermohaline ocean circulation both respond to the climate changes that forced the growth and decline of continental ice sheets. Overall the data show that Nd isotope ratios can be a powerful tool to trace paleocirculation history.
PP31B-04 08:45h
Glacial Nd Isotope Composition of Equatorial Atlantic Bottom Water
Atmospheric and oceanic circulation together redistribute heat and moisture around the Earth. The parallel behavior of climate proxies in ice core and marine sediment records demonstrates that these changes are globally represented. Results from general circulation models attest to the instability of the thermohaline system and its capacity for rapid changes. Nevertheless, the origin of thermohaline instabilities and their relationship to rapid global scale climate events remain unresolved. NADW turnover is expected to dramatically decrease when a sudden flux of fresh water is injected into the North Atlantic. Models differ in detail, but have in common the sensitivity of Atlantic overturning to freshwater injections, and they also produce a cooling of air and sea surface temperatures in the eastern North Atlantic when overturning is slowed or halted. The relative importance of atmospheric teleconnections and changes in the meridional overturning in the North Atlantic require multiple observational and modeling approaches. The approach presented here is the application of Nd isotopes in the ferromanganese fraction of marine sediment cores. Nd isotope ratios in modern Atlantic water profiles follow temperature-salinity variations, indicating that Nd isotope ratios in these waters conserve the characteristics of major water masses. Nd isotope ratios of leached extractions of the authigenic ferromanganese components of core top samples are consistent with modern water values, although the Sr isotope compositions are generally elevated relative to seawater in North Atlantic samples. We present here new data from the western tropical Atlantic. Samples from the last glacial maximum yield significantly more radiogenic values of -7.6 to -8.2 compared to modern values of approximately -12. This approximately 4 epsilon difference is consistent with published Nd isotope evidence from the southeastern Atlantic (Cape Basin) that there was a significant reduction in the NADW contribution to the Atlantic basin during glacial times. Although causes and effects from changing meridional overturning remain open questions, the evidence is mounting for a significant difference in the deep water mixing processes during glacial times compared to today, and is consistent with substantially reduced NADW production.
PP31B-05 09:00h
Stratification and Circulation of the Glacial Ocean: Reconstructing Watermass Geometry and Circulation with Nd Isotopes
One of the most important debates in paleoclimate research is the link between ocean circulation and climate change. On glacial-interglacial timescales, global climate is driven by Milankovich orbital cycles, though the resulting insolation variations are small and require amplifying mechanisms. Changes in the strength of global "conveyor-belt" ocean circulation is one possible amplifying mechanism, and abrupt switches between circulation modes may have triggered rapid climate changes. Understanding the ocean-climate link has been difficult because nutrient-based proxies of ocean circulation disagree with each other. During the last glacial period, benthic foraminiferal carbon isotopes suggest substantially weaker North Atlantic Deep Water (NADW) formation and export, while benthic Cd/Ca indicate a strong flow of North Atlantic -sourced water at intermediate depths. Non-circulatory effects (including fractionation during calcite dissolution, changes in global carbon budget, air-sea gas equilibration, and porewater effects) are known to overprint these proxies, likely causing this discrepancy. Neodymium isotopes can resolve this disagreement because it does not share the same sources of error as nutrient-based proxies. We report Nd isotopes measured on Fe-Mn leaches from sites located throughout the South Atlantic. These sites range in depth from 2000 - 5000 mbsl, allowing a three-dimensional perspective of South Atlantic watermass geometry. The leachates have marine Sr isotopic composition. Coretop samples have Nd isotopic compositions which match overlying bottom waters. This coretop calibration is consistent with known vertical and horizontal geometry of NADW and Antarctic Bottom Water. Samples from the last glacial maximum show a coherent glacial-interglacial pattern of circulation change which is most simply interpreted as indicating a reduction in the amount of NADW reaching the South Atlantic. The relative contribution of NADW to the glacial South Atlantic was less at all depths. The Nd isotopic compositions of Cape Basin sites ( > 2000 mbsl) range from -6.4 to -5.5, indicating that Southern-sourced water dominated all depths. This last glacial profile shows a reduction of vertical stratification of watermass provenance, and suggests that the strong vertical structure apparent in nutrient-based proxy studies may have been caused by depth-dependant non-circulatory effects. Finally, this study places downcore Nd isotope records from the South Atlantic in a consistent framework of glacial-interglacial circulation change.
PP31B-06 INVITED 09:15h
Holocene Paleocirculation from 231Pa/230Th in Sediments from the North Atlantic Ocean
A new record of bulk sedimentary 231Pa/230Th excesses from core GGC5 on the Bermuda Rise has been produced by ICP-MS. The record reveals several aspects of the strength of the meridional overturning circulation during the last ten thousand years. Measurements of uranium, protactinium and thorium in the Holocene section recovered in GGC5 have been resolved at approximately 200-300 years, similar to the response time of excess 231Pa in the water column, and therefore should capture any large departure from modern values in the 231Pa/230Th indicator of paleocirculation. No dramatic increase in ratio or decrease in circulation is indicated by the data, which suggest vigorous overturning in the western basin of the North Atlantic since the end of the Younger Dryas. Instead of large changes in circulation, a series of oscillations punctuates the middle Holocene, indicating a variability of 10-20 percent in the rate of meridional overturning. Specific attention will be paid to millennial events postulated at 8.2 kyr, ~5 kyr, and in the most recent Holocene.
PP31B-07 09:30h
A $^{231}$Pa/$^{230}$Th record of Atlantic Ocean circulation rate during MIS3
$^{231}$Pa/$^{230}$Th has been shown to be a useful kinematic proxy for ocean circulation rate and a valuable complement to nutrient proxies which record 'snapshots' of past water mass configurations. Recent work using $^{231}$Pa/$^{230}$Th indicates stagnation of abyssal waters in the subtropical north Atlantic Ocean between ~17.5 and 15 ka BP, time equivalent to the most recent Heinrich event (McManus et al., 2004). Modeling results indicate a last glacial maximum deep ventilation rate of about 70 percent the modern circulation (Marchal et al., 2000). Questions remain about whether such shifts are typical, and whether changes in circulation rates and deep ocean ventilation are associated with the large-amplitude stadial-interstadial climate variability of the more distant past. A $^{231}$Pa/$^{230}$Th record from the MIS3 interval (~30 to 60 ka BP) on the Blake Outer Ridge (~3400 m core depth) suggests low-amplitude variations in circulation rate related to Dansgaard-Oeschger cycles (defined by correlation of $\delta$$^{18}$O with the GISP2 ice core record). Preliminary data suggest increased circulation vigor during interstadial events relative to stadial events. Heinrich events during this time do not appear to be associated with the dramatic drops in circulation rate seen in the H1 interval on the Bermuda Rise. These new results indicate that there was ventilation of the deep basin through most if not all of MIS3. In contrast to the deeper basin records, an intermediate depth (~1980 m) $^{231}$Pa/$^{230}$Th record from the Carolina Slope indicates greater circulation rates during the LGM than late Holocene; in fact, core-top values from the Carolina Slope are very close to the highest (i.e., lowest rate) values recorded at the deeper Blake Outer Ridge site during MIS3. This depth relationship may indicate more intense intermediate water circulation during glacial times.
PP31B-08 09:45h
Glacial-Interglacial Weathering Rate Variability asRevealed by a High Resolution Laser Ablation Pb Isotope Study of Ferromanganese Crusts
The rate of continental weathering plays a central role in regulating atmospheric carbon dioxide concentration, and hence global climate, on both long and short time scales. Yet the role played by weathering in generating and maintaining the Earths present ice-house conditions is uncertain. We propose that Pb isotopes in ferromanganese crusts offer a fresh perspective on this problem and here we use a new high-resolution technique to resolve changes in the Pb isotope composition of the oceans on a 10-20 kyr timescale. Ferromanganese crusts are faithful recorders of the isotopic composition of oceanic water from which they precipitate. Their slow growth rate (1-10 mm/Myr), however, typically prohibits sampling at an age resolution of less than 200 kyr. Consequently, most studies are concerned with long-term secular changes in isotopic composition. With the advent of laser ablation multi-collector mass spectrometry, however, $ < $30 kyr resolution is readily achievable, even on crusts growing at rates of $ < $2 mm/Myr with little loss in analytical precision. Our data show that over the last 1.5 million years there have been large glacial-interglacial swings (over 1%) in the Pb isotopic composition of the North Atlantic. These swings in isotopic ratio are most readily explained in terms of weathering intensity in the source areas of the Pb through an established relationship between soil age, soil Pb isotopic composition, and cationic weathering flux. We find that during the glacial periods of the last 500 ka, relatively unradiogenic Pb predominates, reflecting a drop in weathering intensity of 1.5-2.5 times, depending on ice volume. When scaled to land area, this decrease offsets the increase in weathering rate thought to occur as a consequence of sea level fall due to ice sheet growth. We suggest that the interplay between these two processes (sea level fall and ice sheet size) ensures that global weathering rates remain constant over short to intermediate (10$^{3}$-10$^{6}$ years) time scales despite large, glacially induced, changes in regional weathering rates.