Piotrowski Alexander M.

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Piotrowski
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Alexander M.
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  • Preprint
    North Atlantic ocean circulation and abrupt climate change during the last glaciation
    ( 2016-05-21) Henry, L. Gene ; McManus, Jerry F. ; Curry, William B. ; Roberts, Natalie L. ; Piotrowski, Alexander M. ; Keigwin, Lloyd D.
    The most recent ice age was characterized by rapid and hemispherically asynchronous climate oscillations, whose origin remains unresolved. Variations in oceanic meridional heat transport may contribute to these repeated climate changes, which were most pronounced during the glacial interval twenty-five to sixty thousand years ago known as marine isotope stage 3 (MIS3). Here we examine a sequence of climate and ocean circulation proxies throughout MIS3 at high resolution in a deep North Atlantic sediment core, combining the kinematic tracer Pa/Th with the most widely applied deep water-mass tracer, δ13CBF. These indicators reveal that Atlantic overturning circulation was reduced during every cool northern stadial, with the greatest reductions during episodic iceberg discharges from the Hudson Strait, and that sharp northern warming followed reinvigorated overturning. These results provide direct evidence for the ocean's persistent, central role in abrupt glacial climate change.
  • Preprint
    The co-evolution of Black Sea level and composition through the last deglaciation and its paleoclimatic significance
    ( 2006-01-28) Major, Candace O. ; Goldstein, Steven L. ; Ryan, William B. F. ; Lericolais, Gilles ; Piotrowski, Alexander M. ; Hajdas, Irka
    The strontium and oxygen isotopic compositions of carbonate shells are a measure of the water delivered to the Black Sea lake since the last glacial maximum. Commencing at ~18 ka BP cal with the arrival of substantial meltwater from the Alpine and northern European ice sheets and overflow via the Caspian Sea from the disintegrating Siberian ice cover, the 87Sr/86Sr ratio rose rapidly from a glacial minima around 0.7087 to reach a set of peaks near 0.7091 in layers of conspicuous reddish-brown clay with a mineralogy of Eurasian provenance. The 87Sr/86Sr ratio oscillates between high in the red-brown layers to low in interbedded gray clays with glacial era mineralogy, indicative that the meltwater came in pulses. On the other hand, the rise of the δ18O ratio from glacial low values of -7 per mil was delayed until15.2 ka BP cal, after the last meltwater pulse. The rising δ18O of the Black Sea lake corresponds with two episodes of calcite precipitation whose interruption corresponds to the Younger Dryas cold period. During each interval of calcite precipitation the δ18O increased a further 2 per mil, without variation in the 87Sr/86Sr composition. During cooling the 87Sr/86Sr ratio trended back toward its glacial value with little change in the δ18O. The disparity between the Sr and O isotope behavior demonstrates that δ18O is not simply a signal of end-member mixing, but instead the δ18O record reflects changes in atmospheric moisture delivered to the Black Sea watershed. At 9.4 ka BP cal the 87Sr/86Sr composition shifted to that of the global ocean and remained there to the present. Since lake water is significantly depleted in strontium relative to seawater, any earlier leakage from the Mediterranean should have left a corresponding signal.
  • Article
    Antarctic intermediate water circulation in the South Atlantic over the past 25,000 years
    (John Wiley & Sons, 2016-10-05) Howe, Jacob N. W. ; Piotrowski, Alexander M. ; Oppo, Delia W. ; Huang, Kuo-Fang ; Mulitza, Stefan ; Chiessi, Cristiano M. ; Blusztajn, Jerzy S.
    Antarctic Intermediate Water is an essential limb of the Atlantic meridional overturning circulation that redistributes heat and nutrients within the Atlantic Ocean. Existing reconstructions have yielded conflicting results on the history of Antarctic Intermediate Water penetration into the Atlantic across the most recent glacial termination. In this study we present leachate, foraminiferal, and detrital neodymium isotope data from three intermediate-depth cores collected from the southern Brazil margin in the South Atlantic covering the past 25 kyr. These results reveal that strong chemical leaching following decarbonation does not extract past seawater neodymium composition in this location. The new foraminiferal records reveal no changes in seawater Nd isotopes during abrupt Northern Hemisphere cold events at these sites. We therefore conclude that there is no evidence for greater incursion of Antarctic Intermediate Water into the South Atlantic during either the Younger Dryas or Heinrich Stadial 1. We do, however, observe more radiogenic Nd isotope values in the intermediate-depth South Atlantic during the mid-Holocene. This radiogenic excursion coincides with evidence for a southward shift in the Southern Hemisphere westerlies that may have resulted in a greater entrainment of radiogenic Pacific-sourced water during intermediate water production in the Atlantic sector of the Southern Ocean. Our intermediate-depth records show similar values to a deglacial foraminiferal Nd isotope record from the deep South Atlantic during the Younger Dryas but are clearly distinct during the Last Glacial Maximum and Heinrich Stadial 1, demonstrating that the South Atlantic remained chemically stratified during Heinrich Stadial 1.