Bernd R. Schöne

Cross-Calibration of Daily Growth Increments, Stable Isotope Variation, and Temperature in the Gulf of California Bivalve Mollusk Chione cortezi: Implications for Paleoenvironmental Analysis

Abstract Annual-oxygen isotope profiles from two live-collected specimens of Chione cortezi Carpenter were analyzed in conjunction with daily growth-increment width profiles and high-resolution temperature records from the same site in the northern Gulf of California. The daily growth-increment profiles serve to date the deposition of the δ18O samples. Then the δ18O values were compared with high-resolution temperature records from the same site. Shell deposition began in late March or early April and ended in late November or early December. δ18O-derived estimates of the maximum and minimum temperature thresholds of growth agree well with those obtained from the dated increment width profile. Shell deposition in these two specimens of C. cortezi from the northern Gulf began when temperature warmed above ∼17°C and slowed or halted when temperature rose above ∼31°C. The temporal resolution of stable isotope samples varies throughout the year. Samples with the coarsest resolution (>3 weeks) were taken from parts of the shell deposited near the minimum and maximum temperature thresholds of growth. Higher resolution samples have intermediate δ 18O values and most represent less than five days of growth. Calculated temperatures from the dated oxygen-isotope samples are similar to observed temperatures. Differences reflect the effects of daily temperature variation, tidal emergence, and enrichment in δ18O of the water in which the clams grew. Stable oxygen-isotope samples used in conjunction with increment-width profiles can provide paleoenvironmental information at sub-weekly to sub-monthly resolution.

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 (δ18O) 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, multi-year δ18O 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 bivalves 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 δ18O 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.

Daily Growth Rates in Shells of Arctica islandica: Assessing Sub-seasonal Environmental Controls on a Long-lived Bivalve Mollusk

Abstract Shells of the extremely long-lived bivalve mollusk Arctica islandica (Linnaeus 1767) provide century-long, multi-proxy records of inter-annual environmental variability in middle- to high-latitude marine settings. Reliable interpretation of these climate archives, however, requires exact knowledge of the length and timing of the growing season and which environmental parameters control shell growth rate during the year. Here, intra-annual growth microstructures, δ18O-derived ambient water temperatures, and δ13C from A. islandica shells collected from the southern and central North Sea are studied. Such data were analyzed in conjunction with observational sea-surface temperature and primary productivity data. Arctica islandica produces circadian growth increments in its shell (on average 31.5 μm per day during age four, measured along the outer shell surface), which allow assignment of calendar dates to each shell portion. The growing season of A. islandica in the upper mixed layer of the ocean (here 25 m water depth) is not continuous over an eight-month period as previously suggested. Rather, it is interrupted during spawning between early September and mid-November. In addition, shell production ceases or is strongly retarded due to food scarcity between mid-December and mid-February. Water temperatures derived from oxygen-isotope ratios are in good accord with observed sea-surface temperatures. In specimens at 25 m water depth, abrupt changes in shell δ18O-derived temperature (Tδ18O) were interpreted to represent vertical displacements of the seasonal thermocline. Daily shell growth is controlled by temperature and food availability. Up to 58% of the variation in daily growth rate is explained by these environmental parameters. This study demonstrates that A. islandica can provide seasonal to subseasonal, precisely dated proxies of environmental variables. Such data are of increasing importance for climate models.

North Atlantic Oscillation dynamics recorded in shells of a long-lived bivalve mollusk

Existing reconstructions of the winter North Atlantic Oscillation (WNAO) are based on terrestrial proxies and historical documents. No direct high-resolution, long-term rec ords from marine settings are available for this major climate-dictating phenomenon, which severely affects a variety of economic aspects of our society. Here we present a 245 yr proxy WNAO index based on shells of the long-lived marine bivalve mollusk Arctica islandica . Variations in annual rates of shell growth are positively correlated with WNAO-related changes in the food supply. Maximum amplitudes in frequency bands of 7–9 and 5–7 yr fall exactly within the range of instrumental and other proxy WNAO indices. These estimates were obtained for specimens collected live, 2000 km apart, in the central North Sea and on the Norwegian Shelf. Hence, the WNAO influences hydrographic regimes of large regions of the ocean. Our study demonstrates that A. islandica can reliably reconstruct WNAO dynamics for time intervals and regions without instrumental records. Our new tool functions as a proxy for the WNAO index prior to the twentieth-century greenhouse forcing and has the potential to further validate other proxy-based WNAO records.

Reconstructing daily temperatures from growth rates of the intertidal bivalve mollusk Chione cortezi (northern Gulf of California, Mexico)

Abstract We establish a model for the reconstruction of average daily sea surface temperatures from calcification rates of an intertidal bivalve mollusk. The rate of shell production in Chione cortezi (Carpenter, 1864, ex Sloat MS) is mainly controlled by water temperature, ontogenetic age and the effect of tidal cycles. Statistical methods developed by dendrochronologists can successfully extract the water temperature signal from daily growth increment chronologies. After removal of noise, the growth rates are expressed as scaled daily growth indices. Average daily water temperatures during the first half of the year are highly correlated with the scaled daily growth index values of recent and subrecent specimens, using the multi-valued function presented here. Increment width analysis can reconstruct daily average water temperatures with a mean error of less than 3%. This technique provides an independent method for reconstructing temperatures in fossil specimens of species with living representatives and can supplement high-resolution temperature reconstructions based on geochemical analysis.

High-resolution Mg/Ca ratios in a coralline red alga as a proxy for Bering Sea temperature variations from 1902 To 1967

Abstract We present the first continuous, high-resolution record of Mg/Ca variations within an encrusting coralline red alga, Clathromorphum nereostratum, from Amchitka Island, Aleutian Islands. Mg/Ca ratios of individual growth increments were analyzed by measuring a single-point, electron-microprobe transect, yielding a resolution of ∼15 samples/year and a 65-year record (1902–1967) of variations. Results show that Mg/Ca ratios in the high-Mg calcite algal framework display pronounced annual cyclicity and archive late spring–late fall sea-surface temperatures (SST) corresponding to the main season of algal growth. Mg/Ca values correlate well to local SST, as well as to an air temperature record from the same region. High spatial correlation to large-scale SST variability in the subarctic North Pacific is observed, with patterns of strongest correlation following the direction of major oceanographic features that play a key role in the exchange of water masses between the North Pacific and the Bering Sea. Our data correlate well with a shorter Mg/Ca record from a second site, corroborating the ability of the alga to reliably record regional environmental signals. In addition, Mg/Ca ratios relate well to a 29-year δ18O time series measured on the same sample, providing additional support for the use of Mg in coralline red algae as a paleotemperature proxy that, unlike algal-δ18O, is not influenced by salinity fluctuations. Moreover, electron microprobe–based analysis enables higher sampling resolution and faster analysis, thus providing a promising approach for future studies of longer C. nereostratum records and applications to other coralline species.

Constructing long-term proxy series for aquatic environments with absolute dating control using a sclerochronological approach: introduction and advanced applications

The possibility of applying absolute dating techniques to annual growth increments from the hard parts of aquatic animals was examined. This was done using the theory of cross-dating, which was adopted from dendrochronological principles. Using two mollusc species as examples, the practical issues of the method were demonstrated. Empirical data were used to evaluate the different time series analysis techniques as follows. Biological growth trends were first captured from original time series using cubic splines. Dimensionless growth indices were obtained by extracting the observed growth values from the values of spline curves as ratios. The common growth signal among the index series was quantified visually and statistically. In statistical analysis, correlations between all possible pairs of indexed sample series and, alternatively, between sample series and master chronology (the average of all other remaining time series) were calculated. It was demonstrated that sample–master correlations were consistently higher than sample–sample correlations. Sclerochronologically cross-dated time series were proved to provide absolute dating of high-resolution proxy records that assessed environmental change in marine and freshwater settings. The wider applicability of the associated techniques is discussed, and it is suggested that use of the term ‘sclerochronology’ be restricted to refer only to material or studies for which careful cross-dating has been successfully applied.

Upstream dams and downstream clams: Growth rates of bivalve mollusks unveil impact of river management on estuarine ecosystems (Colorado River Delta, Mexico)

We studied how the extensive diversion of Colorado River water, induced by dams and agricultural activities of the last 70 years, affected the growth rates of two abundant bivalve mollusk species (Chione cortezi and Chione fluctifraga) in the northern Gulf of California. Shells alive on the delta today (Post-dam shells) grow 5.8–27.9% faster than shells alive prior to the construction of dams (Pre-dam shells). This increase in annual shell production is linked to the currently sharply reduced freshwater influx to the Colorado River estuary. Before the upstream river management, lower salinity retarded growth rates in these bivalves. Intra-annual growth rates were 50% lower during spring and early summer, when river flow was at its maximum. Growth rates in Chione today are largely controlled by temperature and nutrients; prior to the construction of dams and the diversion of the Colorado River flow, seasonal changes in salinity played an important role in regulating calcification rates. Our study employs sclerochronological (growth increment analysis) and geochemical techniques to assess the impact of reduced freshwater influx on bivalve growth rates in the Colorado River estuary. A combination of both techniques provides an excellent tool to evaluate the impact of river management in areas where no pre-impact studies were made. 2003 Elsevier Ltd. All rights reserved.

Reliability of Multitaxon, Multiproxy Reconstructions of Environmental Conditions from Accretionary Biogenic Skeletons

Evaluation and quantification of climate change require data on subseasonal to daily environmental extremes from those periods before instrumental records were available. This study employs a high‐resolution, multitaxon, multiproxy approach and analyzes how faithfully accretionary biogenic skeletons record environmental extremes. Six specimens of two bivalve mollusks (Chione fluctifraga, Mytella guyanensi) and one barnacle species (Chthamalus fissus) from a single habitat (northern Gulf of California, Mexico) were collected. Contemporaneous shell portions from these specimens were analyzed for shell growth rates (sclerochronology) and stable isotopes (δ18O, δ13C) and were compared to instrumental records. The results of these analyses included some significant observations. First, shell δ18O values overestimate winter temperatures and underestimate summer temperatures. Second, the actual diurnal temperature range is not recorded in the biogenic skeletons. Third, skeletal growth is biased toward a species‐specific optimum growth temperature (24°–30.9°C). Therefore, higher sampling resolution will not necessarily capture actual environmental extremes. Despite measured temperature extremes of 37.8° and 4.5°C, none of the studied species recorded temperatures above 30.9° or below 12.2°C. Duration and timing of the annual growing period is species specific as well. Faster shell growth occurred at higher temperatures. Up to 58% (C. fissus) of the variability in shell growth can be explained by water temperature during growth. Contemporaneous trends in shell δ13C show a weak correlation with pigment concentration ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape