Donna Surge

Isotopic evidence for climate change during the Vandal Minimum from Ariopsis felis otoliths and Mercenaria campechiensis shells, southwest Florida, USA

Archaeological evidence from coastal southwest Florida suggests this region and its local inhabitants (the Calusa) were affected by drought and cooling during the Vandal Minimum climate episode (ad 500–800). To test this hypothesis, we reconstructed seasonal-scale climate conditions using stable oxygen and carbon isotope ratios (δ18O and δ13C) preserved in Ariopsis felis otoliths and Mercenaria campechiensis shells. Comparing δ18O records from both species distinguishes between cool versus warm and wet versus dry conditions. δ18O values from four otoliths indicate cooling of winter temperatures occurred in the early Vandal Minimum (ad 500–600) and late half of the middle Vandal Minimum (ad 650–700). Persistent dry summers punctuated by occasional years with wet summers throughout the Vandal Minimum were detected from δ18O values of eight archaeological shells. Our climate reconstructions are in good agreement with archaeological observations and with the cool and dry conditions documented in Europe.

Isotope sclerochronology and season of annual growth line formation in limpet shells (Patella vulgata) from warm- and cold-temperate zones in the eastern North Atlantic

ABSTRACT In the western North Atlantic and Gulf of Mexico, isotope sclerochronology and field studies of the hard clams, Mercenaria mercenaria and M. campechiensis, collected across a latitudinal gradient show patterns in the seasonal timing of slow versus fast shell incremental growth. It is unknown, however, if similar patterns exist in the eastern North Atlantic. The European limpet, Patella vulgata, is abundant in rocky shore communities and archaeological deposits along the eastern North Atlantic. As such, it is a potentially valuable archive for paleoclimate and archaeological research. We used isotope sclerochronology to identify the seasonal timing of annual growth line formation in shells from the cold- and warm-temperate zones and at the boundary between these zones. Four shells from the cold-temperate zone (United Kingdom and Norway), five shells from the warm-temperate zone (Spain), and six shells from the boundary (near the English Channel) were analyzed. The isotopic records represent between two and eight years of growth. Cold-temperate shells formed annual lines in winter, and warm-temperate shells produced annual lines in summer. A mixed pattern was found at the boundary. This pattern in the seasonal timing of slowed growth across a latitudinal gradient is similar to that shown in studies of Mercenaria in the western North Atlantic. Thermal tolerance is the most likely mechanism for the observed changes in the timing of annual growth line formation with latitude.

Isotope sclerochronology of Mercenaria mercenaria, M. Campechiensis, And their natural hybrid form: Does genotype matter?

Abstract Isotopic variation in northern and southern hard clam (quahog) shells is used in studies including paleoecology, paleoclimatology, and archaeology. It is unknown, however, whether species-specific isotopic differences exist. Three genotypes—Mercenaria mercenaria, M. campechiensis, and their natural hybrid form—are found in coastal Florida waters and differentiation of genotypes can be difficult to determine morphologically. This issue may be problematic when using archaeological shells as paleoclimate archives, because genetic analysis cannot be done on such specimens. Their co-occurrence in coastal Florida waters provides a unique opportunity to study whether all three genotypes of modern individuals record the same environmental information preserved as variation in oxygen and stable carbon isotope ratios. A random sample of 49 individuals collected alive at the same time and from the same locality in Pine Island Sound were classified to genotype using allozyme electrophoresis. Three juveniles from each genotype were selected for isotopic analysis to control for ontogenetic effects. Timing of growth increment formation inferred from oxygen isotope ratios reveals similar overall patterns wherein dark (slow growth) increments formed in mid- to late spring and light (fast growth) increments formed in late fall. Results of the mixed model ANOVA (analysis of variance) indicate that no significant species-related differences exist in the variation of oxygen and carbon isotope ratios, although the Kolmogorov-Smirnov goodness-of-fit test detected a systematic difference among δ13C values of M. mercenaria and M. campechiensis comparison and M. mercenaria and the hybrid shell comparison. Any genotype or combination thereof is, thus, suitable for environmental and climate reconstruction using oxygen isotope ratios. The utility of carbon isotope ratios as an environmental proxy, however, remains questionable.

MULTIPROXY SCLEROCHRONOLOGICAL EVIDENCE FOR PLIO-PLEISTOCENE REGIONAL WARMTH: UNITED STATES MID-ATLANTIC COASTAL PLAIN

ABSTRACT Assessments of global Pliocene climate using conventional proxies indicate prevalent warmth at mid- to high latitudes. How this climate change was manifested on a regional basis and on annual timescales remains poorly understood. Oxygen isotope ratios (&dgr;18O) of fossil Mercenaria spp. (bivalve) shells enable reconstruction of growth temperature to estimate sea surface temperature (SST) at seasonal resolution. Comparing growth characteristics of fossil and modern populations potentially provides independent SST constraints. Fossil shells were selected from the Rushmere Member of the Yorktown Formation (Pliocene, early Piacenzian, ∼3.3–3.0 Ma, warm interval) and the Chowan River Formation (Pleistocene, Gelasian, 2.4–1.8 Ma, subsequent cooling) of the U.S. Mid-Atlantic Coastal Plain (MACP). The coldest winter temperatures recorded in the Yorktown Formation shells averaged 17 ± 2 °C and the warmest summer temperatures averaged 25 ± 2 °C. Chowan River Formation shells recorded the coldest winter temperatures averaging 12 ± 2 °C and the warmest summer temperatures averaging 21 ± 2 °C. Modern winter and summer SST is 6 ± 2 °C and 24 ± 2 °C, respectively. The vast majority of data do not approach temperature growth limits of Mercenaria; thus, shells of the Yorktown and Chowan River formations document reduced seasonality relative to modern conditions. Population growth analyses do not provide reasonable quantitative SST estimates. They provide further evidence, however, that fossil populations likely grew in warm-temperate conditions. Isotopic results are consistent with notions of increased meridional heat transport during the Pliocene.

Deciphering annual growth features in Chione elevata shells using isotope sclerochronology

ABSTRACT Chione elevata is a venerid bivalve common in modern molluscan assemblages and in the fossil record throughout the southeastern United States. Knowledge of its life span and growth rates can provide insights into paleoenvironmental and paleoecological conditions for a large number of fossil assemblages. We used isotope sclerochronology to decipher the accretionary record in modern shells of C. elevata. Sixty-three live specimens were collected from a soft-substrate intertidal flat near Wilmington, North Carolina. Aragonite samples drilled from the outer shell layer of three specimens were analyzed for oxygen isotope ratios (&dgr;18O). Previous studies show that dark growth lines in other bivalve taxa from the same geographic area, such as Mercenaria mercenaria, correlated with summer growth cessation. A similar accretionary pattern does not exist in C. elevata. Dark growth lines in C. elevata shells appear randomly distributed. Their occurrence does not always correspond with maxima in &dgr;18O values (winter growth cessation). Additionally, dark growth lines are not correlated with concentric ornaments on the surface of the shell. Instead, maxima in &dgr;18O values correspond to indentations, or notches, on the surface of the shell. We characterize these concentric external notches as annual markers of winter growth cessation.

Application of Molluscan Analyses to the Reconstruction of Past Environmental Conditions in Estuaries

Molluscs possess a number of attributes that make them an excellent source of past environmental conditions in estuaries: they are common in estuarine environments; they typically have hard shells and are usually well preserved in sediments; they are relatively easy to detect in the environment; they have limited mobility as adults; they grow by incremental addition of layers to their shells; and they are found in all the major environments surrounding estuaries—terrestrial, freshwater, brackish, and marine waters. Analysis of molluscan assemblages can contribute information about past changes in sea level, climate, land use patterns, anthropogenic alterations, salinity, and other parameters of the benthic habitat and water chemistry within the estuary. High-resolution (from less than a day to annual) records of changes in environmental parameters can be obtained by analyzing the incremental growth layers in mollusc shells (sclerochronology). The shell layers retain information on changes in water temperature, salinity, seasonality, climate, river discharge, productivity, pollution and human activity. Isotopic analyses of mollusc shell growth layers can be problematic in estuaries where water temperatures and isotopic ratios can vary simultaneously; however, methods are being developed to overcome these problems. In addition to sclerochronology, molluscs are important to Holocene and Pleistocene estuarine palaeoenvironmental studies because of their use in the development of age models through radiocarbon dating, amino acid racemization, uranium-thorium series dating, and electron spin resonance (ESR) dating.

Shellfish Exploitation in the Western Canary Islands Over the Last Two Millennia

ABSTRACT The residents of the Canary Archipelago consumed limpets since the arrival of humans ∼2500 yrs. ago, and these harvested gastropods were deposited in large coastal shell middens. This work preliminarily explores shell margin oxygen isotope composition (δ18O) and body size of the black limpet (Patella candei d’Orbigny, 1840) from archaeological sites in the Canary Islands to assess possible seasonal variability and intensity of shellfish collection throughout the late Holocene. The shell margin δ18O values of 100 shells (radiocarbon dated between ∼500 and ∼1800 cal. yr BP) were analysed to estimate sea surface temperature (SST) at time of death. Paleotemperature estimates suggest shellfish harvesting was not year-round, and was avoided in the cooler months (when SST < 20°C). This pattern differs from most higher latitude Mesolithic and Neolithic human groups, which gathered shellfish year-round, targeting winter more heavily. Preliminary body-size measurements suggest shell sizes have experienced a decline from aboriginal times to the present, which possibly resulted from increasing anthropogenic pressures. During aboriginal inhabitation, maximum adult shell size remained stable, suggesting that present-day harvesting practices are more intense than harvesting from aboriginal human groups. This intensive collection has likely diminished the average adult size of limpet populations in the islands by ∼27%.