USING SUB-FOSSIL AND HISTORIC NAUTILUS TO TEST POTENTIAL LINKS BETWEEN CALCIUM ISOTOPE FRACTIONATION AND PCO2

The calcium isotope system (δ^44/40Ca) has been used to investigate connections between global weathering and carbonate sediment fluxes on timescales of tens-of-thousands to millions of years. High temporal resolution (<100 kyr duration) analyses of carbonate rocks may reveal transient flux imbalances, variations in the proportion of calcite and aragonite composing carbonate sediment, or mechanistic changes in the carbonate fractionation factor due to ocean acidification. Biogenic carbonates, which can be clearly assessed for diagenetic alteration, may offer unbiased recorders of these perturbations or alternatively, could respond through varying vital effects. To date, few studies have attempted calibrations of biogenic carbonates precipitated under variable pCO₂ conditions in natural systems.

To test whether δ^44/40Ca values of molluscan carbonate respond to pCO₂ variations, we used a high-precision (±0.04‰; 2SD) double-spike thermal ionization mass spectrometry method to measure δ^44/40Ca values of a series of Nautilus macromphalus from New Caledonia spanning the last 5,000 years. Nautilus macromphalus occur in relatively shallow water depths of 0-750 m around the island and therefore only record local conditions or global conditions that propagate locally and do not reflect any long-distance migratory signal. Individuals sampled span pre-industrial, early-industrial, and recent pCO₂ values ranging from 280-400 ppm. Initial results show measurable differences in δ^44/40Ca values (+0.2‰) of shell precipitated under low (5 kyr) and high (early 2000’s) pCO₂ levels. These results may indicate reduction in the magnitude of the carbonate fractionation due to increasing pCO₂, increases in carbonate precipitation over the last 5 kyr, or reduction in the apparent trophic level of Nautilus due to ecosystem restructuring near New Caledonia.