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Applying clumped isotope paleothermometry in sclerochronology studies: a brief tutorial for D47-curious sclerochronologists

Session: Timestamped Biomineralized Structures in Coastal Environmental Monitoring and Cultural Research

Presenting Author:

Sierra Petersen

Authors:

Petersen, Sierra V.¹, Gomes, Lucas², Quizon, Alex³, Kim, Erin H⁴

(1) Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA; Museum of Paleontology, University of Michigan, Ann Arbor, MI, USA, (2) Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA, (3) Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA, (4) Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA,

Abstract:

An ongoing challenge to d¹⁸O-based paleothermometry is that carbonate d¹⁸O is not solely dependent on temperature, but also reflects the oxygen isotopic composition of surrounding water (d¹⁸O_w). In coastal oceans where many mollusks live, modern data demonstrates that d¹⁸O_w can vary both spatially and temporally. This weakness of carbonate d¹⁸O as a proxy is therefore especially acute in the field of sclerochronology, where variations in d¹⁸O_carb within a single shell are frequently converted into seasonal temperature ranges assuming no subannual variations in d¹⁸O_w, and seasonal temperature extremes are highly dependent on the absolute d¹⁸O_w value assumed. Clumped isotope paleothermometry avoids these issues because the measured quantity (D₄₇) is a function of temperature alone, allowing the calculation of formation d¹⁸O_w when D47-derived temperature (T-D47) is combined with d¹⁸O_carb. However, compared to d¹⁸O_carb, the higher associated labor and time required to make a D47 measurement and the larger (apparent) uncertainty in D47-derived temperature have made this new proxy undesirable to some researchers.

This presentation will describe different methods of combining d¹⁸O_carb sclerochronology with D47 analysis and discuss the pros and cons of each. We will present examples of the simplest clumped isotope sampling scheme (d¹⁸O_c sclerochronology + bulk, life-averaged D47), an intermediate complexity scheme (d¹⁸O_c sclerochronology + seasonally-targeted D47), and the most intensive clumped sampling scheme (simultaneous D47 + d¹⁸O_c sclerochronology). Through these examples, we will emphasize how the addition of D47 provides different results than those derived from d¹⁸O_c alone. Examples presented will include scallops (Miocene Carolinapecten from Virginia, Pleistocene Carolinapecten from Florida), clams (Last Interglacial Mercenaria from Massachusetts), and cockles (modern Dinocardium from Florida and South Carolina) from a variety of locations and time periods.