Formula omitted] and [formula omitted] ratios in skeletal calcite of Mytilus trossulus: Covariation with metabolic rate, salinity, and carbon isotopic composition of seawater

Abstract

Minor element and isotopic compositions of marine bivalve calcite are frequently used as proxy records of seawater temperature and salinity. Although molluscan calcite is secreted at or near oxygen isotope equilibrium, the influence of metabolic activity (i.e., vital effects) on skeletal SrCa ratios and δ 13C values is not well known. We present measurements of skeletal chemistry from (a) consecutive samples milled in chronological order from the organism's final year of growth and (b) adjacent samples within three separate growth bands of the marine mussel Mytilus trossulus to investigate chemical disequilibrium effects among different parts of the shell.Seawater temperature and salinity were monitored for one year at Squirrel Cove, British Columbia. At the end of the year, a young, rapidly growing mussel (mussel A) and an old, slowly growing mussel (mussel B) were harvested from this site. Growth bands within the shells were sampled to provide a chronological record of shell carbonate chemistry. Results from consecutive samples show (1) a significantly higher average SrCa ratio in mussel A than that in mussel B and (2) δ 13C values that correlate well with salinity in mussel A but not in mussel B. Results from time-equivalent, adjacent samples show (1) higher SrCa and δ 13C values near the ventral margin than at time-equivalent regions on lateral margins of the shell and (2) little or no variation in δ 18O values.These observations suggest that skeletal chemistry (SrCa and δ 13C) is primarily controlled by rate of mantle metabolic activity and secondarily modified by variation in seawater salinity. Because metabolic activity in the mantle at the site of carbonate precipitation varies with shell curvature, the composition of calcite secreted along lateral margins is influenced to a greater extent by metabolic activity than calcite secreted coevally at the central margin of the shell. Hence, the chemistry of calcite secreted at the ventral margin is precipitated in near-equilibrium with seawater and allows accurate estimation of seawater SrCa ratios and carbon isotopic composition. In contrast, shell precipitation along lateral margins is dominantly controlled by metabolic activity, and calcite chemistry is not in equilibrium with ambient seawater. In this context, we present a biomineralization model where variations in skeletal SrCa and δ 13C values are explained in the context of mantle metabolic activity and seawater salinity.