Trends in Seawater Boron-based Proxies during the Late Paleocene and Early Eocene Associated with Long-term Warming

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; 56Ma, an abrupt global warming and ocean acidification event) reveal a decline of ~0.3 in the pH of the mixed-layer [1], an anomaly that is well within the range of estimates based on the observed shoaling of the carbonate compensation depth (CCD) [2, and references therein]. The PETM occurred superimposed on a long-term warming trend that initiated in the Late Paleocene and continued into the Early Eocene (LPEE; 53-59Ma). The magnitude of warming [3] and deepening of the CCD [4] indicate that the LPEE was driven by a rise in pCO₂ nearly equivalent to that of the PETM [5].

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 20kyr resolution B/Ca record shows a long-term decline of ~25% during the LPEE, as well as subtle 400kyr 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 δ¹¹B_seawater 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.

[1] Penman et al. 2014. Paleoceanography.

[2] Palike et al. 2012. Nature.

[3] Zachos et al. 2001. Science.

[4] Leon-Rodriguez and Dickens 2010. Palaeogeogrphy, Palaeoclimatology, and Palaeoecology.

[5] Komar, Zeebe and Dickens 2013. Paleoceanography.