Long- and short-term trends in seawater B/Ca and pH across the Paleocene-Eocene boundary

Author(s): Harper, D. T.; Penman, D. E.; Hoenisch, B.; Zachos, J. C.
Author Affiliation(s): Primary:
University of California Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, CA, United States
Other:
Columbia University, Department of Earth and Environmental Sciences, Palisades, NY, United States
Volume Title: AGU 2013 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2013; American Geophysical Union 2013 fall meeting, San Francisco, CA, Dec. 9-13, 2013. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Boron-based proxies in planktonic foraminifers show promise in reconstructing past pH and carbonate chemistry of the surface oceans because test shell boron incorporation is highly controlled by seawater pH, borate ion concentration, and bicarbonate ion concentration, and thus pCO2. The Paleocene-Eocene Thermal Maximum (PETM), an abrupt global warming event characterized by a large carbon isotope excursion and seafloor carbonate dissolution, occurred during a long term warming trend from the late Paleocene (≈59 Ma) into the early Eocene (≈53 Ma). The PETM has been associated with a relatively quick release of a large volume (thousands of petagrams) of carbon to the atmosphere as methane and/or carbon dioxide, which in theory should have temporarily reduced surface ocean pH globally. This has recently been verified with measurements of negative anomalies in both B/Ca and B isotopes of planktonic foraminifera at ODP Site 1209 in the subtropical North Pacific (Penman et al., in review). The anomalies are most consistent with a pH decline of between 0.2 to 0.4. To place this anomaly in the context of long-term late Paleocene to early Eocene trends, and to characterize any orbitally paced signals in ocean pH, here we apply ≈20 kyr resolution B/Ca measurements and ≈200 kyr resolution boron isotope measurements in surface dwelling planktonic foraminifers from the same Pacific site, 1209. A long-term reduction in surface dwelling foraminifer B/Ca of ≈20-25% is observed over 3 Ma from the late Paleocene to the early Eocene. This might reflect a rise in pCO2, and/or a trend in B/Ca of seawater. We also find low amplitude, low-frequency 400 kyr eccentricity cycles of B/Ca, reflecting changes in the ocean pH due orbitally-forced carbon-cycle fluctuations, with B/Ca varying by approximately 10-15% on these time scales (ΔpH <0.1). Both the long-term trend and low-frequency cycles are consistent with expected changes in pCO2 and pH as inferred from other records (i.e., carbon isotopes, wt.%CaCO3, deep sea temperature).
Year of Publication: 2013
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Carbon dioxide; Cenozoic; Leg 198; North Pacific; Northwest Pacific; ODP Site 1209; Ocean Drilling Program; Pacific Ocean; Paleo-oceanography; Paleocene-Eocene Thermal Maximum; Paleoclimatology; Paleogene; Shatsky Rise; Stratigraphic boundary; Tertiary; West Pacific
Coordinates: N323900 N324000 E1583100 E1583000
Record ID: 2014051754
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