Trends in seawater boron-based proxies during the late Paleocene and early Eocene associated with long-term warming

Author(s): Harper, D. T.; Penman, D. E.; Hoenisch, B.; Zachos, J. C.
Author Affiliation(s): Primary:
University of California Santa Cruz, Santa Cruz, CA, United States
Lamont-Doherty Earth Observatory, United States
Volume Title: AGU 2014 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2014; American Geophysical Union 2014 fall meeting, San Francisco, CA, Dec. 15-19, 2014. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Boron isotopes (δ11B) and boron/calcium ratios (B/Ca) in tests of planktic foraminifera are controlled by equilibrium reactions between boron and carbon species in seawater, and thus represent important proxies of past marine carbonate chemistry. Indeed, the recent application of these boron-based proxies to fossil shells of planktic foraminifera from cores spanning the Paleocene-Eocene Thermal Maximum (PETM; 56 Ma, an abrupt global warming and ocean acidification event) reveal a decline of ∼0.3 in the pH of the mixed-layer, an anomaly that is well within the range of estimates based on the observed shoaling of the carbonate compensation depth (CCD). The PETM occurred superimposed on a long-term warming trend that initiated in the Late Paleocene and continued into the Early Eocene (LPEE; 53-59 Ma). The magnitude of warming and deepening of the CCD indicate that the LPEE was driven by a rise in pCO2 nearly equivalent to that of the PETM. Here we extend the PETM record of boron-based proxies at IODP Site 1209 across the LPEE, in conjunction with stable carbon and oxygen isotopes in planktic foraminifera, in order to better constrain the long-term changes in pH and carbonate chemistry that accompanied the suggested rise in atmospheric CO2. The 20 kyr resolution B/Ca record shows a long-term decline of ∼25% during the LPEE, as well as subtle 400 kyr cycles associated with eccentricity that mirror those observed in δ13C, and thus might reflect on changes in pH. The lower resolution δ11B record exhibits little change during the Late Paleocene before decreasing step-wise to lower values following the PETM, indicating that either pH in the upper ocean did not change significantly prior to the PETM, despite warming and inferred pCO2 increase, or changes in δ11Bseawater compensated for pH driven changes. As verification of these observations at Site 1209, complementary B/Ca and δ11B records are being generated for Atlantic IODP Sites 1262 and 1263.
Year of Publication: 2014
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Atlantic Ocean; Cenozoic; Leg 208; ODP Site 1262; ODP Site 1263; Ocean Drilling Program; Paleocene; Paleogene; South Atlantic; Tertiary; Upper Paleocene; Walvis Ridge
Coordinates: S271100 S271100 E0013500 E0013400
S283200 S283200 E0024700 E0024700
Record ID: 2015117269
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