Foram Farming: Calibration of the Foraminiferal Paleo-Thermometer to Estimate Deep-Time Glacioeustasy

Abstract

Foraminiferal Mg/Ca has been used effectively as a proxy for Quaternary seawater temperatures, which have been combined with Foraminiferal d18O to yield records of paleo-seawater d18O—a proxy for glacioeustasy on glacial/interglacial time scales. When extending this method to deeper timescales, like those useful to the development of petroleum reservoirs, it is important to take into account the secular variation in seawater Mg/Ca and its effect on the partitioning of magnesium into foraminiferal tests. Four studies over the last decade have aimed to reduce this uncertainty by culturing foraminifera in waters of varying Mg/Ca and temperature; those studies used lower-Mg calcite to intermediate-Mg calcite foraminifera. This study reports on success in establishing a long-term colony of the high-Mg calcite (HMC) benthic foraminifer, Peneroplis planatus, from Abu Dhabi, UAE. Specimens are kept in independent reservoirs that are attached to a 650 L recirculating system utilizing artificial seawater. Peneroplids from this system were placed in experimental conditions (seawater Mg/Ca from 1 to 8 at 25°C) for 42 days, each growing three to five new chambers marked by calcein. They were harvested and analyzed via electron microprobe. Mg/Ca of newly grown chambers gradually changed to reflect experimental conditions. Foram Mg/Ca vs. seawater Mg/Ca was best fit by the function: Foram Mg/Ca = 0.05 (Seawater Mg/Ca) ^ 0.56 (where Foram Mg/Ca is in mol/mol). This model is very similar to the only other model reported for a HMC benthic foraminifer, Operculina ammonoides. If coupled with foraminiferal d18O, past sea level can be calculated by estimating paleo-seawater d18O, a proxy for global ice volume. This could be done not only with Cenozoic and Mesozoic foraminifera, but also for times without surviving oceanic crust. For example, geochemical data from fusulinids, an extinct clade of Paleozoic large benthic foraminifera similar to the peneroplids grown in this study, could be used to estimate sealevels in the Paleozoic using this calibration. An improved Permian sealevel curve could could reduce uncertainty when building reservoir models of Permian hydrocarbon reservoirs.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016