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The cause of late Cenozoic mass extinction in the western Atlantic: insights from sclerochronologyHeavy late Cenozoic extinction amongst marine molluscs in the western Atlantic has traditionally been interpreted as a consequence of climatic deterioration. However, the pattern of extinction was not the same in the eastern Atlantic, where conditions also became colder. A fall in primary productivity, suggested by a decline in phosphate deposition, may be the real explanation for western Atlantic extinctions. Evidence in support comes from isotopic- and increment-based (sclerochronological) indications of growth rate in Pliocene scallops. A western Atlantic genus that has survived to the present (Placopecten) had the same moderate growth rate in the Pliocene as now, while two genera that became extinct (Carolinapecten and Chesapecten) had growth rates as fast as any known amongst living scallops. Such rapid growth implies abundant food. Selective extinction of a fast-growing species has also been documented amongst Pliocene oysters in the Caribbean region and attributed to a decline in primary productivity. The likely cause of this is the development of the Central American Isthmus and the consequent reorganization of oceanic circulation in the Gulf of Mexico and wider North Atlantic.
Environment and extinction in the late Cenozoic of the North Atlantic area - insights from sclerochronologySclerochronology is the sister-field of dendrochronology, the study of tree rings. It involves analysis of the size and composition of increments within mineralised skeletons formed by accretion, such as those of bivalve molluscs, corals and coralline algae. Detailed information can be obtained on the age, growth-rate and environment of the organism concerned. In this talk I will show how sclerochronology is providing insights into Plio-Pleistocene changes in marine productivity and temperature in the North Atlantic region: their nature, cause and possible links to bivalve mass extinction during this interval (45% and 65% loss of species in the eastern and western Atlantic, respectively). This information may inform prediction of future extinction associated with global warming.
Evidence, cause and consequence of exceptionally rapid growth in Pliocene scallops of the US eastern seaboardScallops are amongst the fastest growing of bivalves, with many species growing in the order of 40 mm per annum in early ontogeny, and reliable evidence of early ontogenetic growth-rates up to 70 mm per annum in wild populations of certain species. From the evidence of oxygen isotope (δ18O) sclerochronology, modern examples of the western Atlantic genus Placopecten grow about 40 mm per annum in early ontogeny. The same approach reveals similar growth rates in Pliocene examples from Virginia and North Carolina. By contrast Pliocene examples of the extinct genera Chesapecten and Carolinapecten from Virginia grew at least 75 mm per annum, faster than has been recorded in any wild modern scallop, and examples of Carolinapecten from Florida grew up to 140 mm per annum, twice the maximum rate in wild modern scallops. The rapid overall growth of Carolinapecten is matched by exceptionally large microgrowth increments. In specimens with the fastest overall growth the number of increments approximately equals the number of days indicated by the oxygen isotopic data (e.g. c. 180 over half an oxygen isotope cycle), implying that deposition was daily. Specimens with slower overall growth do not have smaller increments but have substantially fewer than the number of days indicated by δ18O evidence, showing that reduced overall growth was a consequence of periodic interruptions rather than permanently less favourable conditions. Since few individuals lived more than a year, rapid somatic growth must have been accompanied by gamete production, implying abundant food resources. Intervals of particularly rapid growth (largest microgrowth increments) are fairly closely correlated with increases in δ13C, as might have been caused by phytoplankton blooms. However, there is little evidence that these were stimulated by upwelling since there are few indications of a matching increase in δ18O (i.e. colder water). Instead, primary productivity may have been enhanced by nutrient supply from the land. Whatever the cause of high primary productivity in the Pliocene of the US eastern seaboard, the subsequent demise of two scallop genera with exceptionally rapid growth seems as likely to relate to a decline in productivity as to a fall in temperature.