Boron in Foraminiferal Calcite as an Indicator of Seawater Carbonate Chemistry

• Main Author: Allen, Katherine Ann
• Language: English
• Published: 2013
• Subjects: Paleoclimatology
• Online Access: https://doi.org/10.7916/D8ZK5PWD

Foraminifera are unicellular organisms with a wide marine distribution. Many species secrete carbonate tests whose physical and chemical nature reflect the seawater conditions in which they grow. Thus, fossil tests preserved on the sea floor represent an archive that may be used to investigate the composition of ancient seawater. With the aim of improving our understanding of past ocean-climate links, I have tested proxies for seawater composition in the modern ocean and applied them to a key period in Earth history. The ratio of boron to calcium (B/Ca) in the calcite tests of planktic foraminifers has previously been suggested as a proxy for past seawater carbonate chemistry, but controls on B incorporation are not yet clear. The theoretical basis for this proxy is rooted in the pH-dependent concentration of dissolved borate (B(OH)_4-) and its subsequent incorporation into foraminiferal calcite. In this thesis, I present: 1) new insights into the environmental controls on B/Ca revealed by culture experiments with living foraminifers, and 2) new reconstructions of past seawater chemistry during the last deglaciation based on B/Ca of fossil calcite from deep sea sediments. To test environmental controls on B incorporation, I performed several culture experiments that quantified the effects of pH, temperature, salinity, dissolved boron and inorganic carbon concentrations on the calcite tests of the planktic foraminifer species O. universa, G. sacculifer, and G. ruber (pink). In these experiments, B/Ca increases with pH (lower [HCO_3-], higher [CO2_3-] and [B(OH)_4-]) and salinity, but not with temperature. Thus, normalizing B/Ca data to a constant salinity (e.g., S=35) should improve our ability to isolate the carbonate chemistry signal in B/Ca paleo-records and samples from different ocean sites. In addition, B/Ca decreases with total dissolved inorganic carbon (DIC) at constant pH (higher [HCO_3-] and [CO2_3-], constant [B(OH)_4-]), which suggests competition between aqueous boron and carbon species for inclusion into the calcite lattice. While different cultured species exhibit similar B/Ca behavior in response to salinity, temperature, and pH changes, their absolute B/Ca values are offset under identical seawater conditions. Thus, B/Ca is both a function of environmental parameters that exert strong influence on test composition as well as biological processes that result in species offsets. To determine whether these culture calibrations are applicable in the open ocean, I used equations relating the B/Ca of cultured foraminifers with experimental seawater properties to predict B/Ca of wild specimens derived from sediment core-tops. Most measured core-top values for O. universal and G. sacculifer are similar to values predicted by culture calibrations (average offsets are 4 and 15 μmol mol^-1, respectively) but values predicted for coretop G. ruber deviate by up to 60 μmol mol^-1 from predicted values. The greater discrepancy observed for core-tops may suggest that our experiments still fall short of identifying all environmental controls on B/Ca and/or that we need to revisit the growth conditions assumed for planktic foraminifers, in particular the depth habitat of G. ruber. Further, an evaluation of planktic foraminiferal downcore data shows that B/Ca in planktic foraminifers is not sufficiently sensitive to surface ocean carbonate chemistry to permit reconstruction of Pleistocene atmospheric CO2 changes. However, B/Ca may serve as a useful proxy in environments that experienced large carbonate system changes, such as upwelling zones, or large events such as those during the Paleocene-Eocene. In contrast to planktic foraminifers, B/Ca of benthic foraminifer tests appears to respond to deep-water carbonate saturation state (ΔCO2_3-). B/Ca of the benthic species Cibicidoides wuellerstorfi increases linearly with ΔCO2_3- in all major ocean basins, as demonstrated in the modern core-top calibration of Yu and Elderfield (2007). To gain insight into carbon storage in the deep ocean across the last glacial termination, I investigated the B/Ca composition of C. wuellerstorfi in a sediment core from New Zealand's Bay of Plenty, located at a depth of 1,627 meters. The resulting reconstruction indicates that ΔCO2_3- changed up to 30 μmol kg^-1 across the deglaciation. Combined with benthic δ^13C and independent paleo-O_2 estimates, the [CO2_3-] record indicates increased storage of CO_2 in the deep ocean during the LGM, with major roles for the biologic pump and carbonate compensation.