A Diagenetic Origin for δ18O Variability on the Margins of the Great Bahama Bank, Insights From Clumped Isotopes

Abstract

The sediments deposited at ODP Site 1003 on the margin of Great Bahama Bank (GBB) exhibit a range of chemical and isotopic compositions. The mineralogy at this site consists predominantly of low-Mg calcite (LMC) and in the upper portion (< 200 mbsf) High Magnesium Calcite (HMC) and Aragonite. The aragonite and HMC are derived predominantly from the Bank itself, while the LMC is derived from pelagic sources and that produced by neomorphism of aragonite and HMC and recrystallization of biogenic LMC. The variability of the carbonate δ18O presents an interesting challenge. Modern sediments at Site 1003 has a composition between -0.5‰ and 0‰, which over the upper 200m increases to +2.0‰. This corresponds to a decrease in aragonite abundance and a consequent increase in (LMC). Below 200 mbsf there is a trend back towards negative δ18O values, not accompanied with any mineralogical change. Here we investigate these two trends using clumped isotopes (Δ47) to determine if these are primary “mixing model” effects or a result of carbonate diagenesis. Porewater chemistry is extremely sensitive to the alteration of carbonates, changes too small to detect via chemical or isotopic measurement of the sediments can make significant changes to the pore fluid. Here we present porewater data, which provide evidence for diagenesis at this site. Strontium ion concentration gradients provide a minimum estimate for diagenetic rate, as they reach the saturation state of Celestite (SrSO4) in the upper 200m of sediment. Additionally we present porewater δ18O and Sr87/Sr86 of pore fluids, these two measurements provide evidence for deeper diagenesis at this site. The chemical processes present in the subsurface at ODP Site 1003 demonstrate that the co-evolution of sediments and porewaters occurs over a range of temperatures due to geothermal heating, necessitating independent verification of temperature. The precision of this method is constantly improving and as more facilities are able to measure clumped isotopes of carbonates, it could become an invaluable tool for understanding systems such as these.

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