Douglas S. Jones

Ecological and paleoenvironmental information using stable isotope profiles from living and fossil molluscs

Stable oxygen and carbon isotopic profiles from Recent specimens of Spisula solidissima and Placopecten magellanicus collected off the Virginia coast are used to calibrate annual temperature-controlled cycles in the δ18O profile and shell growth characteristics. The amplitude of the annual δ18O cycles decreases with increasing water depth, and may be used as a relative depth indicator. The isotopic of profiles of two fossil specimens of Pleistocene Spisula are very similar in amplitude and character to modern analogues, and verify the formation of annual growth increments in these fossil specimens. Offsets between the δ18O profiles of the fossil and modern specimens are related to known temperature and ice-volume effects during the late Pleistocene and support the utility of mollusc-isotope records as a stratigraphic tool. n nCarbon isotopic profiles from all specimens show light values in the spring and/or fall, associated with seasonal phytoplankton productivity. Offsets in the carbon isotopic composition between infaunal and epifaunal species may be related to sediment porewater geochemistry and oxidation of organic matter. Trends toward lighter δ13C values through ontogeny suggest the effects of metabolic changes from a fast-growing juvenile into a slower-growing, sexually-mature adult. Anomalies in both the oxygen and carbon isotopic profiles identify the formation of minor shell growth increments as corresponding to hydrographic perturbations and to winter cold temperatures.

Annual cycle of shell growth increment formation in two continental shelf bivalves and its paleoecologic significance

The bivalves Spisula solidissima , the Atlantic surf clam, and Arctica islandica , the ocean quahog, from the continental shelf off New Jersey, contain repeating structures in their shells. By analyzing the growing shell margins in living specimens at bi-weekly (or sometimes monthly) intervals throughout two consecutive years, it was possible to define an annual cycle of shell growth increment formation in both species. The shell increments in each species are microstructurally distinct units that form over a period of several months at select seasons of the year. Each species has two alternating shell growth increments, GI I and GI II. GI I (the annual growth line of previous studies) is formed annually in the late summer-fall in S. solidissima and in the fall-early winter in A. islandica. These periods correspond to the spawning phase of the reproductive cycle in both species. No winter rings were found. The annual increments were used to determine age and growth rate in both Recent and Pleistocene specimens. They may also be useful in determining season of death. Because shell growth increments are formed in synchrony among living populations in these species, mass mortalities may be distinguished in the fossil record. Accurate age and growth rate determinations in fossils are important in many paleobiologic contexts, such as deciding between increased longevity or growth rate in cases of phyletic size increase.

Growth history and ecology of the Atlantic surf clam, Spisula solidissima (Dillwyn), as revealed by stable isotopes and annual shell increments☆

Abstract The shells of the Atlantic surf clam, Spisula solidissima (Dillwyn), contain a record of both life history and environmental changes. These shell records were investigated using oxygen and carbon stable isotopic analyses (δ18O, δ13C) and shell growth increment analyses. δ18O variations across annual shell increments reflect the yearly cycle of sea-water temperatures off the New Jersey coast, further documenting the proposed annual periodicity of the major shell increments. The 11-yr shell record analyzed here confirms that shell growth is most rapid in spring-early summer, slow in late summer-fall, and extremely slow or non-existent in winter. Shell growth appears to occur in isotopic equilibrium with sea water and measured δ18O values are used to refine the aragonite-water temperature scale. Variations in the timing of annual growth increment formation are noted as well as ontogenetic effects upon the range of isotopic values recorded in shell carbonate. Both the δ18O and δ13C profiles are influenced by changes in the sea-water temperature regime over the 11-yr period studied (1965–1976) and record these effects in the shell. The combination of stable isotope and growth increment analyses provides a powerful tool for interpreting the shell records of both modern and fossil molluscs.

Seasonal temperature-salinity changes and thermocline development in the mid-Atlantic Bight as recorded by the isotopic composition of bivalves

Stable isotope records across annual growth increments in specimens of the surf clam Spisula solidissima from the mid-Atlantic Bight shelf from 10 m and 45 m depths reflect the changes in temperature and nutrient concentrations on the shelf over the year. The δ 18 O and δ 13 C records from clams at the two depths record well-mixed conditions in the water column during the winter months and the development of a thermocline during the summer. Spring high productivity and a transient salinity excursion in surface waters are also recorded. Reconstructing the paleoceanography of late Cenozoic temperate continental shelves may be possible using stable isotope records from fossil Spisula solidissima and other bivalves.

Life History of Symbiont-Bearing Giant Clams from Stable Isotope Profiles

Stable isotopic and shell-growth banding studies of the symbiont-bearing giant clam Tridacna maxima reveal the existence of two growth phases related to sexual maturity that can be discerned in the shells of extinct and extant mollusks. The changeover from the first to second growth phase at an age of approximately 10 years is accompanied by a decrease in rate of calcification and suggests a reordering of energy priorities between biomineralization and reproduction. The carbon-13 to carbon-12 ratio of Tridacna maxima is systematically depleted relative to symbiont-barren mollusks, making it possible to determine the importance of algal-molluscan symbiosis to the functional morphology and paleoecology of mollusks in the geologic record.

Primary productivity and the Cretaceous/Tertiary boundary event in the oceans

Abstract Geochemical and paleontological evidence indicate that marine primary productivity decreased rapidly at the Cretaceous-Tertiary boundary resulting in the selective elimination of those organisms directly dependent upon the flux of organic matter as a food source (filter and suspension feeders). Detritus and deposit feeders, however, suffered relatively fewer extinctions, apparently utilizing the reservoir of organic matter stored within the sediments. Lower rates of oceanic productivity might have continued for at least 1.5 m.y. following the initial decrease despite the rapid evolution of fauna and flora during the early Paleocene. Although these results can be viewed as being compatible with the bolide impact hypothesis, the extended period of low productivity afterwards suggests some longer term effects on the biosphere than predicted by such a model.

Stable isotopic investigation of physiological and environmental changes recorded in shell carbonate from the giant clam Tridacna maxima

The aragonitic shell of the photosymbiont-bearing bivalve Tridacna maxima contains a record of the physiological and environmental changes the organism has experienced during its lifetime. This record is preserved as chemical and microstructural variations throughout the shell. Stable isotopic analyses of oxygen (18O/16O) and carbon (13C/12C) in shell carbonate were combined with growth increment studies to interpret the shell record of specimens collected from the Rose Atoll (Lat. 14°31′S; Long. 168°10′W) in April 1982. The seasonal water temperature cycle is recorded in the oxygen isotopic signature of the clams, permitting the recognition of annual cycles in the δ18O profile. The total number of these cycles corresponds to the age of a specimen, while the cycle length is a measure of the yearly growth rate. Large-amplitude cycles, reflecting year-round calcification, characterize the early portion of the growth record. With the onset of sexual maturity and slower growth at an age of approximately ten years, the cycles decrease in amplitude and become more erratic. During this later growth phase calcification is limited to the cooler months of the year, perhaps in response to a re-ordering of energy priorities between growth and gametogenesis. A growth curve developed from the δ18O profile indicates rapid juvenile shell growth followed by slower growth thereafter producing a lifespan of several decades. Carbon isotopic analyses of T. maxima were compared to analyses of the symbiont-barren gastropod Terebra areolata collected from the same locality in April 1984. A 2‰ depletion in the δ13C composition of T. maxima shell carbonate is attributed to a symbiontenhanced metabolic rate and an increased flow of isotopically light, respired CO2 into the carbon pool used in calcification. Such a depletion may prove useful in identifying the presence of photosymbionts in extinct species of fossil mollusks.

Origin of the epeirogenic uplift of Pliocene-Pleistocene beach ridges in Florida and development of the Florida karst

Marine fossils of Pleistocene age are known to occur in beach ridges near the border of northern Florida and southern Georgia at elevations of between 42 and 49 m above mean sea level. No evidence exists for a massive melt-off of glacial ice, which would be required to raise sea level to these elevations. Florida, therefore, must have been uplifted epeirogenically during the Pleistocene. Measurement of dissolved solids in Florida9s springs demonstrates that the karst area is losing a minimum of 1.2 × 10 6 m 3 /yr of limestone through spring flow, the equivalent of 1 m of surficial limestone every 38,000 yr. This loss has led to an isostatic uplift of the north-central part of the Florida peninsula of at least 36 m during Pleistocene and Holocene time, which agrees with observed elevations of marine terraces.

Biotic, geochemical, and paleomagnetic changes across the Cretaceous/Tertiary boundary at Braggs, Alabama

Exposed near Braggs, Alabama, is one of the few well-studied, nearly continuous shallow-marine Cretaceous/Tertiary boundary sections; it allows a glimpse of the biotic and environmental changes that occurred in the latest Cretaceous to earliest Paleocene. Paleomagnetic, strontium isotopic, and biostratigraphic data closely constrain the age of a series of lithologic, geochemical, and biotic variations and suggest that no more than 100–200 ka could be missing at the boundary. A major reduction in macrofaunal diversity associated with lithofacies changes occurs prior to but within 300 ka of the nannofossil-defined boundary. Approximately 40% of the apparent faunal reduction is attributed to the “Lazarus effect.” Faunal and floral assemblages, trends in carbon isotopic composition of benthic invertebrates, and lithologic characteristics indicate that a latest Maestrichtian regression culminated near the boundary (Chron C29R; Micula murus zone), significantly later than recent estimates. Water depths at this site remained shallow during the subsequent early Paleocene (zone NP1) transgression and did not reach depths equivalent to those of the late Maestrichtian until zone NP2. Relatively minor climatic changes across the boundary are suggested by a ≤4 °C cooling trend seen in the oxygen-isotope paleotemperatures. A high-resolution 87 Sr/ 86 Sr record from well-preserved macrofossil calcite shows a pattern of smooth variation and elevated values near the boundary; however, the early Paleocene “spike” of other workers was not found.

History and Development of the Marine Invertebrate Faunas Separated by the Central American Isthmus

Throughout the vast majority of Tertiary time (almost 65 million years) there was no Panama land bridge, and North and South America were detached from one another. This condition can be traced back to the initial separation of the North American and South American (still attached to Africa) plates in the Jurassic, about 165 million years ago (Pindell and Dewey, 1982). Following the onset of rifting and continental break-up in the early Mesozoic, it is uncertain as to the amount of time during which North and South America were actually isolated. The reconstructions of Pindell and Dewey (1982) and Donnelly (Chapter 4, this volume) suggest an early isolation, shortly after 165 m.y.a. Anderson and Schmidt (1983), however, propose a different scenario for the evolution of middle America, which would have juxtaposed blocks preserving land contact between North and South America much longer, perhaps into the late Cretaceous. Regardless of the preferred reconstruction, most authors agree that no land bridge existed for at least 70 m.y., from the late Cretaceous up until contact was reestablished in the Plio-Pleistocene. The establishment of the Panama land bridge has been well documented from considerations of the interchange between terrestrial faunas arising on both continents (e.g., Simpson, 1940, 1953; Webb, 1976; Marshall et al., 1979, 1982; and others in this volume), although the precise timing of this event is more poorly known. The migration route is also uncertain as some have argued for migration via the Nicaraguan Rise rather than through present-day Guatemala (Durham, 1985).