Date of Award

Spring 5-11-2018

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Katherine Allen

Second Committee Member

Brenda Hall

Third Committee Member

Elisabeth Sikes

Abstract

Sediment cores from New Zealand’s Bay of Plenty and the Chatham Rise in the Southwest Pacific were sampled to establish a regional Mg/Ca–temperature calibration for the benthic foraminifer Uvigerina peregrina. Comparison of foraminiferal Mg/Ca from core-top sediments to local bottom water temperatures reveals a Mg/Ca–temperature relationship broadly consistent with previously published calibrations. In addition to bottom water temperatures, other environmental parameters are examined for possible influence on the Mg/Ca of foraminiferal calcite. Elderfield et al. (2006) proposed that such parameters may exert an influence at colder temperatures, particularly below temperatures of ~3oC (e.g. Lear et al., 2002; Elderfield et al., 2006; Bryan and Marchitto, 2008). Multiple cores, from water depths between 2400 and 3300 meters, yielded unexpectedly high Mg/Ca ratios (~1.35 mmol/mol) given the ambient bottom water temperatures of ~2oC.

Several non-temperature influences were ruled out as the possible cause of the high Mg/Ca measurements. These include morphotype variations, dissolution, down-slope transport, inorganic calcite overgrowths, and carbonate ion saturation. Further analyses, including SEM/EDS analysis of the interior of test chambers, revealed the presence of aluminosilicate crystals growing within chambers of U. peregrina. It is hypothesized that authigenic aluminosilicate growths may present difficulties in measuring and interpreting Mg/Ca in infaunal foraminifera by contributing additional cations during analysis. This affects sediments from New Zealand, and may also occur in other locations. The chemical conditions required for such aluminosilicate precipitation are expected to include high detrital sediment inputs (i.e. fluvial or volcanic), intense surface ocean productivity (i.e. high biogenic silica flux), and high dissolved inorganic carbon concentrations in the water mass overlying the seafloor.

Due to the widespread use of the benthic Mg/Ca paleotemperature proxy, it is important to identify any factors that could potentially complicate its use or interpretation. The observed aluminosilicate grains emphasize the need to look beyond the carbonate system when considering diagenetic influences on foraminiferal calcite. Factors such as pore water chemistry and sediment type could exert a significant influence on Mg/Ca measurements, potentially overwriting the Mg/Ca–temperature signal of foraminiferal calcite. Identifying the presence of silicate contaminants and the conditions that cause them to precipitate could enable a better understanding of the factors leading to Mg/Ca measurements that reflect factors other than primary temperature control. Our results suggest that SEM imaging of foraminifer chamber interiors, particularly in areas susceptible to aluminosilicate precipitation, should be done as part of the sample selection process, both in core-top sediments and in paleo records, to ensure production of quality Mg/Ca datasets without diagenetic overprinting.

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