Resolution and Fidelity of Oxygen Isotopes as Paleotemperature Proxies in Bivalve Mollusk Shells: Models and Observations

Abstract

Bivalve mollusks are biological chart recorders: their shells contain a record of environmental conditions in the form of geochemical variation. However, these records are often incomplete. Growth cessations and/or changing growth rates can reduce the range and resolution of the recorded environmental conditions.To investigate the effects of these variables on geochemical profiles, stable oxygen isotope (␦ 18 O) profiles were modeled using several growth parameters. Two sets of profiles were calculated: one with constant daily increment widths, the other based on the annual pattern of daily increment width variation observed in the northern Gulf of California bivalve mollusk Chione cortezi. In both sets of models, multiyear ␦ 18 O profiles were calculated assuming that the bivalve shell grows continuously throughout its life. Other profiles were calculated to simulate an ontogenetic decrease in growth rate by decreasing the growth period, daily growth rate, or both. Altering the growth period simulates the effects of thermal thresholds, above or below which no shell material is deposited. Decreasing the daily growth rate results in lower annual shell growth rates while keeping the growth period constant. Combining the two provides a more accurate representation of bivalve shell growth in many subtropical and temperate species.In addition to the modeling exercise, the shell of a Chione cortezi that lived in the northern Gulf of California was sampled in two ways. First, low-resolution (300 micron) samples were recovered from the entire growth profile along the axis of maximum shell height (umbo to the commissure). Second, high-resolution (50 micron) samples were taken from regions of the shell representing winter growth from late in the bivalve's life.Modeling results and observations indicate that the fullest range of environmental conditions only is reflected in the earliest years of growth; profiles from successive years have reduced amplitudes, sample resolutions, or both. Variation of intra-annual growth rate in models simulating continuous growth can produce cuspate ␦ 18 O profiles that mimic shutdowns. More detailed sampling in later stages of ontogeny can reconstruct a fuller range of environmental conditions. Finally, within-shell trends in isotopic amplitudes and averages may reflect decreases in growth rate rather than environmental fluctuations. Therefore, particular care should be taken when interpreting inter-annual isotope profiles from long-lived species.

Mesolithic shellfish exploitation in SW Italy: seasonal evidence from the oxygen isotopic composition of Osilinus turbinatus shells

“…2c) (Schifano and Censi, 1983;Mannino et al, 2008). Reduction and interruption, of shell secretion increase the time-averaging of d 18 O in shell carbonate samples (Goodwin et al, 2003 (Mannino et al, 2008 ranging from 21 to 25 C (mean offset from À0.1 to À0.8 C), and not provide temperature over 25 C in summer (mean offset of À2 C compared to measured SSTs) (Fig. 4).…”

Increased seasonality in the Western Mediterranean during the last glacial from limpet shell geochemistry

“…The four Patella shells record between 82 and 107% of the observed range in monthly average SST around Gibraltar (~7.5°C). Regular sample intervals are used when milling the shells so if growth slows or ceases for an interval due to biological factors, such as age (Sato, 1995), or environmental factors, such as temperature (Goodwin et al, 2001), then larger amounts of time are averaged (Goodwin et al, 2003). The range in SST recorded by each shell must therefore be considered a minimum.…”

Oxygen isotope ratios in the shell of <i>Mytilus edulis</i>: archives of glacier meltwater in Greenland?

“…Bulk shell composition is shown as the range of data in a box and whisker diagram ( (Fig. 4a-j) (Goewert et al, 2007;Goodwin et al, 2003).…”