Bones of recent mammals in the Amboseli Basin, southern Kenya, exhibit distinctive weathering characteristics that can be related to the time since death and to the local conditions of temperature, humidity and soil chemistry. A categorization of weathering characteristics into six stages, recognizable on descriptive criteria, provides a basis for investigation of weathering rates and processes. The time necessary to achieve each successive weathering stage has been calibrated using known-age carcasses. Most bones decompose beyond recognition in 10 to 15 yr. Bones of animals under 100 kg and juveniles appear to weather more rapidly than bones of large animals or adults. Small-scale rather than widespread environmental factors seem to have greatest influence on weathering characteristics and rates. Bone weathering is potentially valuable as evidence for the period of time represented in recent or fossil bone assemblages, in- cluding those on archeological sites, and may also be an important tool in censusing populations of animals in modern ecosystems.

Taphonomic and Ecologic Information Form Bone Weathering

The potential for sea ice-albedo feedback to give rise to nonlinear climate change in the Arctic Ocean – defined as a nonlinear relationship between polar and global temperature change or, equivalently, a time-varying polar amplification – is explored in IPCC AR4 climate models. Five models supplying SRES A1B ensembles for the 21 st century are examined and very linear relationships are found between polar and global temperatures (indicating linear Arctic Ocean climate change), and between polar temperature and albedo (the potential source of nonlinearity). Two of the climate models have Arctic Ocean simulations that become annually sea ice-free under the stronger CO 2 increase to quadrupling forcing. Both of these runs show increases in polar amplification at polar temperatures above-5 o C and one exhibits heat budget changes that are consistent with the small ice cap instability of simple energy balance models. Both models show linear warming up to a polar temperature of-5 o C, well above the disappearance of their September ice covers at about-9 o C. Below-5 o C, surface albedo decreases smoothly as reductions move, progressively, to earlier parts of the sunlit period. Atmospheric heat transport exerts a strong cooling effect during the transition to annually ice-free conditions. Specialized experiments with atmosphere and coupled models show that the main damping mechanism for sea ice region surface temperature is reduced upward heat flux through the adjacent ice-free oceans resulting in reduced atmospheric heat transport into the region.

Geophysical fluid dynamics.

To better interpret the isotopic composition of ancient aragonitic fossils, stable-isotopic analyses have been performed on live and modern specimens of aragonitic foraminifera, gastropods and scaphopods. Samples were collected from the continental margins off southern California and Texas, U.S.A., and Mexico, and provide a range in ambient temperature of 2.6–22.0°C.

We observed a strong covariance between the δ18O of the aragonitic foraminifera Hoeglundina elegans and that of coeval aragonitic mollusks. On the average, Hoeglundina was 0.2 ± 0.2‰ depleted in 18O relative to the mollusks, and 0.6 ± 0.3‰ enriched relative to the calcitic foraminifera Uvigerina. This enrichment in 18O of aragonite relative to calcite is similar to that observed in previous experimental and theoretical studies. The temperature dependences of mollusk and Hoeglundina δ18O-values were not notably different from that previously determined for inorganically precipitated calcite, and no significant temperature dependence in Hoeglundina-Uvigerina18O fractionation was observed.

Of note is the temperature dependence of the δ13C of the biogenic aragonite. Relative to the dissolved inorganic carbon (DIC), the δ13C of Hoeglundina and the mollusks decreased by 0.11 and 0.13‰, respectively, per °C increase in temperature. The temperature dependence in Hoeglundina-DIC 13C enrichment, and the lack of it in Uvigerina-DIC enrichment, accounts for the temperature dependence in Hoeglundina-Uvigerina (calcitic) fractionation noted by us and previous workers. Isotopic differences between coeval specimens of these genera provide a rough measure of paleotemperature without requiring a knowledge of the isotopic composition of the paleo-ocean.

Oxygen and carbon isotope fractionation in biogenic aragonite: temperature effects

Diverse bilaterian clades emerged apparently within a few million years during the early Cambrian, and various environmental, developmental, and ecological causes have been proposed to explain this abrupt appearance. A compilation of the patterns of fossil and molecular diversification, comparative developmental data, and information on ecological feeding strategies indicate that the major animal clades diverged many tens of millions of years before their first appearance in the fossil record, demonstrating a macroevolutionary lag between the establishment of their developmental toolkits during the Cryogenian [(850 to 635 million years ago (Ma)], and the later ecological success of metazoans during the Ediacaran (635 to 541 Ma) and Cambrian (541 to 488 Ma) periods. We argue that this diversification involved new forms of developmental regulation, as well as innovations in networks of ecological interaction within the context of permissive environmental circumstances.

https://science.sciencemag.org/content/sci/334/6059/1091.full.pdf

The Cambrian conundrum: Early divergence and later ecological success in the early history of animals

13C and 18O isotopic disequilibrium in biological carbonates: I. Patterns

/pdf/magnetite-biomineralization-and-magnetoreception-in-1kdcap4sch.pdf

Magnetite biomineralization and magnetoreception in organisms

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

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 ana- lyzing 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 I. 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.

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

Most bivalve shells contain macroscopic growth-increment patterns which form with an annual periodicity. These are best observed in radial shell cross-sections and consist of alternating dark (translucent in transmitted light) and light (opaque in transmitted light) increments. A couplet of dark and light increments constitutes one year's shell growth and subdivides the continuous shell growth record into successive, annual components. Conventional wisdom suggests that dark increments represent slow shell growth associated with cold winter temperatures. Such a pattern characterizes shells of the much-studied hard clam, Mercenaria mercenaria, from coastal New England. Investigations of other species indicate, however, that not all dark increments form in response to cold temperatures. Yearly cycles in oxygen isotopic data from fossil and modern bivalves demonstrate considerable variability in the season of growth increment formation. Isotopic data suggest Jurassic Gryphaea arcuata from Great Britain formed dark increments during the warmest season, as do many other bivalves. In fact, oxygen isotope profiles across the shells of modern and fossil Mercenaria spp. from the US. east coast and the Atlantic and Gulf Coastal Plains indicate that in northern latitudes the dark increment typically forms during the winter season whereas in lower latitudes it forms during the summer to early fall. A transition zone occurs in the Middle Atlantic region where aspects of both patterns are observed. Water temperature extremes (high or low) appear to be the primary factor controlling the timing of dark (translucent) increment formation. Because of this dichotomy in the season of growth-increment formation, high-resolution isotopic sampling is invaluable when interpreting shell records in sclerochronological contexts, particularly for fossil bivalves.

Douglas S. Jones

Papers

Magnetite biomineralization and magnetoreception in organisms

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

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

Marking time with bivalve shells; oxygen isotopes and season of annual increment formation

Magnetite Biomineralization and Magnetoreception in Organisms : a New Biomagnetism Vol.5