High-resolution grain size analysis and its significance for detecting ocean acidification at the onset of the Paleocene-Eocene Thermal Maximum (PETM; 55Ma)

The Paleocene-Eocene Thermal Maximum (PETM; 55Ma) is widely considered a close ancient analog to modern global warming. A host of recent investigations have elucidated the scale and nature of the climate forcing during the PETM, as well as the range of atmospheric, oceanographic and biotic impacts. Introduction of massive amounts of greenhouse gases into the ocean-atmosphere system at the onset of the event is known to have led to abrupt shoaling of the lysocline and calcite compensation depth in the oceans as observed at deep-sea locations by a marked increase in the dissolution of calcareous microfossils and correspondingly sharp lithologic changes. The occurrence of surface ocean acidification during the initial stages of the PETM is not documented largely because the potential evidence is overprinted by pervasive dissolution at the sea floor. We present detailed grain size analysis from a high-resolution sample set across the PETM at Ocean Drilling Program Sites 690 (Maud Rise, Southern Ocean), 1209 (Shatsky Rise, Pacific Ocean) and 1262 (Walvis Ridge, South Atlantic Ocean) and at the Wilson Lake drill hole from the New Jersey coastal plain. The Wilson Lake section is dominated by clastic material, thus samples were processed to obtain the grain size distribution of the carbonate fraction. Grain size data were collected using a Malvern Mastersizer, an instrument that optically measures particle size between 0.1 and 1000 micrometer in diameter. The results show dramatic differences is size trends between sites that are consistent with their depths with respect to the CCD and lysocline. At the same time, the base of the PETM is characterized by very sharp changes in grain size distribution at Site 1262, where dissolution is most severe and progressively less abrupt changes at Site 1209, Site 690 and Wilson Lake. This progression is consistent with known differences in the magnitude of the lysocline and CCD shoaling at these sites. Comparison of grain size, carbonate and stable isotope data produces more accurate estimates of the depth of carbonate “burn down” at Sites 1209 and 1262. At the other sites, comparison of nannofossil and benthic foraminiferal preservation across the base of the PETM allows us to evaluate whether there was a brief period of surface-water acidification prior to the onset of deep-water acidification. For all sites, grain size data provide more quantitative estimates of the changes in flux of planktonic foraminifera and nannoplankton during the course of the PETM. Nannoplankton dominate the carbonate flux at all sites except Site 690 where the event is marked by complex pattern of variation in foraminiferal flux.

Bralower, T. J., Kump, L., Eccles, L., Smith, G. J., Lindemann, T. L., Bowen, G. J., Schneider Mor, A. & Thomas, E., 2010. High-resolution grain size analysis and its significance for detecting ocean acidification at the onset of the Paleocene-Eocene Thermal Maximum (PETM; 55Ma). American Geophysical Union, Fall Meeting 2010, abstract #PP13E-05. Abstract.

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