Perspectives on the diagenetic alteration of marine carbonates using a multi-proxy approach: A multi-site comparison of Mg, Ca, and Sr isotopic compositions of carbonates

Open Access
Author:
Chanda, Piyali
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 03, 2016
Committee Members:
  • Matthew Scott Fantle, Dissertation Advisor
  • Matthew Scott Fantle, Committee Chair
  • Timothy Bralower, Committee Member
  • Michael Allan Arthur, Committee Member
  • Christopher Aaron Gorski, Outside Member
Keywords:
  • Mg isotopes
  • diagenesis
  • marine carbonates
  • Ca isotopes
  • Sr isotopes
  • geochemical proxy
Abstract:
Geochemical proxy-based reconstructions widely utilize the trace elemental and isotopic compositions of marine biogenic carbonates to interpret past climatic and oceanographic conditions. However, such proxy-based reconstructions are often challenged by marine diagenesis as carbonates are highly susceptible to diagenetic alteration, especially to partial dissolution and calcite recrystallization. Thus, quantifying diagenetic effects is a prerequisite for the development of any carbonate-based geochemical proxy. Among the metal isotopes, Mg isotopic composition (δ26Mg) of foraminiferal carbonates is a promising proxy to reconstruct secular seawater δ26Mg variability, which is useful in understanding long-term changes in the Mg geochemical cycle. However, Mg in marine carbonates is particularly susceptible to diagenetic alterations due to its low abundance in carbonates compared to pore fluids. Hence, the development of δ26Mg of marine carbonates as a proxy demands a thorough understanding of the isotopic systematics of Mg in marine sedimentary systems, which to date has not been accomplished. The primary goal of this dissertation is to quantify the effect of diagenesis on metal isotopes, in particular, the δ26Mg of marine carbonates from various depositional settings. Multiple marine sites are investigated to evaluate the effect of advection, diffusion, and lithological variation within the sedimentary column on the extent of calcite diagenesis. The first two chapters are focused on comparing the extent of diagenetic alteration among various marine sites and quantifying the impact of such diagenetic reactions on carbonate-based proxies. In the first study, the effect of diagenetic recrystallization on bulk carbonate δ26Mg is quantified from an open marine site (ODP Site 1171, Hole A), which is influenced by upward advection. Trace elemental (e.g., Mg/Ca and Sr/Ca) and isotopic compositions (δ26Mg and 87Sr/86Sr) of pore fluids and bulk carbonates are analyzed from Hole 1171A using an ICP-AES and a Neptune Plus MC-ICP-MS, respectively. A systematic decrease in bulk carbonate δ26Mg (-0.8 to -2.0 ‰) with depth is observed, which was not entirely explained by the changes in nannofossil and foraminiferal assemblages in the bulk sediments documented by smear slide observations. Application of a 1-D depositional reactive transport model to the pore fluid and bulk carbonate chemistry demonstrates that calcite recrystallization can account for the observed downcore shift in carbonate δ26Mg. Additionally, downcore trends in 87Sr/86Sr, Mg/Ca, and Sr/Ca ratios of bulk carbonates also correlate well with the inferred diagenetic shift in the carbonate δ26Mg. In the second chapter, the δ26Mg, δ44/40Ca, and 87Sr/86Sr ratios of pore fluids and bulk carbonates are analyzed from three marine carbonate-rich sites (ODP Holes 762B, 806B, and 807A) of comparable age that are not influenced by advection. The purpose of this study was to evaluate the impact of a diffusive lower boundary and intracolumn lithological variability on the extent of carbonate diagenesis and its impact on the preservation of geochemical proxies such as carbonate δ26Mg. Two adjacent holes (Hole 807A and 806B) are studied to evaluate the impact of a variable diffusive lower boundary on the calcite recrystallization rate and its influence on carbonate δ26Mg. Numerical modeling of Ca and Sr geochemistry suggest that the carbonates from Hole 806B have experienced higher recrystallization rates (~5%/Ma) compared to the Hole 807A (~2%/Ma). The diagenetic shift in bulk carbonate δ26Mg at both sites is - 0.5 to -1.0‰ over the 800 meters. The investigation of Hole 762B demonstrates the potential of pore fluid δ26Mg to diagnose the increasing abundance of clays in the carbonate-rich sediments. An increase in bulk carbonate δ26Mg by ~ 0.6 ‰, along with increase in Mg/Ca, Sr/Ca, and Na/Ca ratios within the clay-rich layer indicate that the presence of clay enhances the preservation of proxy archives. Therefore, this study significantly contributes towards the understanding of the major controls on marine pore fluids from carbonate-rich sediments. In the final chapter, the exchange rates of foraminiferal calcite recovered from the Cariaco Basin sediment traps and from deep marine sediment cores (ODP Hole 807A) are quantified through a series of batch exchange experiments using a radioactive 45Ca tracer technique. As foraminiferal calcites are extensively utilized in geochemical proxy-based reconstruction, quantifying the exchange rates helps to evaluate the extent of recrystallization on foraminferal geochemical proxies. The exchange experiments suggest the modern foraminiferal tests exchange faster (6.50 ∙10-4 to 0.30 ∙10-4 mol/m2/d) than the fossil foraminferal tests (2.50 ∙10-4 to 0.40 ∙10-4 mol/m2/d). The increase in [Ca2+], [Mg2+], and [Sr2+] in the fluid of a set of parallel non-tracer experiments indicates that dissolution-precipitation is the dominant mechanism of the atom exchange in foraminiferal calcite. Interestingly, in the presence of aqueous silica in some of the reactors the exchange rate of modern foraminifera decreases (3.70∙10-4 to 0.13∙10-4 mol/m2/d). This observation indicates that the presence of dissolved silica in the system can inhibit calcite-fluid exchange, which enhances the preservation of foraminiferal tests and minimizes the extent of alteration of foraminiferal proxies.