Cooler winters as a possible cause of mass extinctions at the eocene/oligocene boundary

The Eocene/Oligocene boundary, at about 33.7 Myr ago, marks one of the largest extinctions of marine invertebrates in the Cenozoic period(1). For example, turnover of mollusc species in the US Gulf coastal plain was over 90% at this time(2,3). A temperature change across this boundary-from warm Eocene climates to cooler conditions in the Oligocene-has been suggested as a cause of this extinction event(4), but climate reconstructions have not provided support for this hypothesis. Here we report stable oxygen isotope measurements of aragonite in fish otoliths-ear stones-collected across the Eocene/Oligocene boundary. Palaeotemperatures reconstructed from mean otolith oxygen isotope values show little change through this interval, in agreement with previous studies(5,6). From incremental microsampling of otoliths, however, we can resolve the seasonal variation in temperature, recorded as the otoliths continue to accrete new material over the life of the fish. These seasonal data suggest that winters became about 4 degrees C colder across the Eocene/Oligocene boundary. We suggest that temperature variability, rather than change in mean annual temperature, helped to cause faunal turnover during this transition.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62707/1/407887a0.pd

Composition of the early Oligocene ocean from coral stable isotope and elemental chemistry

A sectioned and polished specimen of the coral Archohelia vicksburgensis from the early Oligocene Byram Formation (∼30 Ma) near Vicksburg, Mississippi, reveals 12 prominent annual growth bands. Stable oxygen isotopic compositions of 77 growth-band-parallel microsamples of original aragonite exhibit well-constrained fluctuations that range between −2.0 and −4.8. Variation in Δ 18 O of coral carbonate reflects seasonal variation in temperature ranging from 12 to 24 °C about a mean of 18 °C. These values are consistent with those derived from a bivalve and a fish otolith from the same unit, each using independently derived palaeotemperature equations. Mg/Ca and Sr/Ca ratios were determined for 40 additional samples spanning five of the 12 annual bands. Palaeotemperatures calculated using elemental-ratio thermometers calibrated on modern corals are consistently lower; mean temperature from Mg/Ca ratios are 12.5 ± 1 °C while those from Sr/Ca are 5.8 ± 2.2 °C. Assuming that Δ 18 O-derived temperatures are correct, relationships between temperature and elemental ratio for corals growing in today's ocean can be used to estimate Oligocene palaeoseawater Mg/Ca and Sr/Ca ratios. Calculations indicate that early Oligocene seawater Mg/Ca was ∼81% (4.2 mol mol −1 ) and Sr/Ca ∼109% (9.9 mmol mol −1 ) of modern values. Oligocene seawater with this degree of Mg depletion and Sr enrichment is in good agreement with that expected during the Palaeogene transition from ‘calcite’ to ‘aragonite’ seas. Lower Oligocene Mg/Ca probably reflects a decrease toward the present day in sea-floor hydrothermal activity and concomitant decrease in scavenging of magnesium from seawater. Elevated Sr/Ca ratio may record lesser amounts of Oligocene aragonite precipitation and a correspondingly lower flux of strontium into the sedimentary carbonate reservoir than today.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72914/1/j.1472-4677.2004.00025.x.pd

Dimensions of Sedimentary Lithotopes and Taxonomies of Fishes

The size of subgroups among larger taxonomic units, as measured by the number of taxa within them, is a metric of fundamental importance to the appreciation of causes of change in biodiversity in both time and space. Central to such evaluations is an understanding of the expected and observed variation in the numbers and sizes of groups comprising various taxonomic levels. Here we show that numbers of fish taxa within subdivisions (memberships) of any supertaxon in a Linnaean taxonomy are virtually identical to areas of patches of like sediment (lithotopes) that are distributed across various depositional surfaces. Both sedimentary surfaces and Linnaean taxonomies are closely approximated by functions that generally describe random divisions of geographic and/or shape-space. We describe a ‘broken plate’ model for taxonomic membership that is akin to Robert MacArthur’s (1957) classical ‘broken stick’ model for abundance distributions, where species abundances in an ecosystem are described by an exponential function of abundance (segment length) frequencies reflecting the random subdivision of resources. In a taxonomic context, the broken plate presumes that the amount of morphospace realized at any taxonomic level is proportional to the numbers of subtaxa of which it is comprised. A hypothetical transect across the morphospace associated with any higher taxon would comprise a ‘broken stick’, or exponential, distribution of square roots of the number of contained subtaxa. Taxonomic membership (occupied morphospace) within the higher taxon is therefore randomly partitioned among subtaxa, analogous to the sizes of fragments of the broken plate. Thus, just as the broken stick distribution is well-described using only the length of the stick and the number of segments into which it is broken, the partitioning of taxa into subtaxa within any supertaxon is random and adequately described using only the number of taxa and the number of subtaxa into which they are partitioned. Such ‘broken plate’ functions yield excellent agreement for membership partitioning among classes, orders, families, and genera of fishes. Quantification across all taxonomic levels provides several insights related to the biodiversity of this important group: (1) Membership of taxonomic groups of fishes is self-similar among all levels of Linnaean division (e.g., families per order, genera per family, species per genus) and is almost entirely independent of levels of taxonomic separation between groups being considered, with an average of seven to eight members within any taxonomic group. (2) The ‘broken plate’ representation implies that divisions within one taxonomic level are independent of all other divisions; a similar partitioning of species among genera belonging to both diverse and depauperate families supports the supposition that little ‘memory’ exists between levels of taxonomic membership. (3) Special explanations for the generation of apparently extreme polytype may be largely unnecessary; taxonomic diversities expected from the ‘broken plate’ model suggest that observed disparity in numbers of fish species comprising many clades is no greater or less than one would expect from a random fragmentation of morphospace.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/171083/1/UMMZ MP 209 Vol. 3 12.23.pdf-1Description of UMMZ MP 209 Vol. 3 12.23.pdf : Main articleSEL

Pronounced zonal heterogeneity in Eocene southern high-latitude sea surface temperatures

Paleoclimate studies suggest that increased global warmth during the Eocene epoch was greatly amplified at high latitudes, a state that climate models cannot fully reproduce. However, proxy estimates of Eocene near-Antarctic sea surface temperatures (SSTs) have produced widely divergent results at similar latitudes, with SSTs above 20 °C in the southwest Pacific contrasting with SSTs between 5 and 15 °C in the South Atlantic. Validation of this zonal temperature difference has been impeded by uncertainties inherent to the individual paleotemperature proxies applied at these sites. Here, we present multiproxy data from Seymour Island, near the Antarctic Peninsula, that provides well-constrained evidence for annual SSTs of 10–17 °C (1σ SD) during the middle and late Eocene. Comparison of the same paleotemperature proxy at Seymour Island and at the East Tasman Plateau indicate the presence of a large and consistent middle-to-late Eocene SST gradient of ∼7 °C between these two sites located at similar paleolatitudes. Intermediate-complexity climate model simulations suggest that enhanced oceanic heat transport in the South Pacific, driven by deep-water formation in the Ross Sea, was largely responsible for the observed SST gradient. These results indicate that very warm SSTs, in excess of 18 °C, did not extend uniformly across the Eocene southern high latitudes, and suggest that thermohaline circulation may partially control the distribution of high-latitude ocean temperatures in greenhouse climates. The pronounced zonal SST heterogeneity evident in the Eocene cautions against inferring past meridional temperature gradients using spatially limited data within given latitudinal bands

Climate Change and Trophic Response of the Antarctic Bottom Fauna

BACKGROUND: As Earth warms, temperate and subpolar marine species will increasingly shift their geographic ranges poleward. The endemic shelf fauna of Antarctica is especially vulnerable to climate-mediated biological invasions because cold temperatures currently exclude the durophagous (shell-breaking) predators that structure shallow-benthic communities elsewhere. METHODOLOGY/PRINCIPAL FINDINGS: We used the Eocene fossil record from Seymour Island, Antarctic Peninsula, to project specifically how global warming will reorganize the nearshore benthos of Antarctica. A long-term cooling trend, which began with a sharp temperature drop approximately 41 Ma (million years ago), eliminated durophagous predators-teleosts (modern bony fish), decapod crustaceans (crabs and lobsters) and almost all neoselachian elasmobranchs (modern sharks and rays)-from Antarctic nearshore waters after the Eocene. Even prior to those extinctions, durophagous predators became less active as coastal sea temperatures declined from 41 Ma to the end of the Eocene, approximately 33.5 Ma. In response, dense populations of suspension-feeding ophiuroids and crinoids abruptly appeared. Dense aggregations of brachiopods transcended the cooling event with no apparent change in predation pressure, nor were there changes in the frequency of shell-drilling predation on venerid bivalves. CONCLUSIONS/SIGNIFICANCE: Rapid warming in the Southern Ocean is now removing the physiological barriers to shell-breaking predators, and crabs are returning to the Antarctic Peninsula. Over the coming decades to centuries, we predict a rapid reversal of the Eocene trends. Increasing predation will reduce or eliminate extant dense populations of suspension-feeding echinoderms from nearshore habitats along the Peninsula while brachiopods will continue to form large populations, and the intensity of shell-drilling predation on infaunal bivalves will not change appreciably. In time the ecological effects of global warming could spread to other portions of the Antarctic coast. The differential responses of faunal components will reduce the endemic character of Antarctic subtidal communities, homogenizing them with nearshore communities at lower latitudes

Temperature, seasonality and salinity history of the early Eocene North Sea Basin inferred from fish otoliths and mollusks

status: publishe

Investigations on fossil bivalves and wood from Seymour Island, Antarctica

Quasi-periodic variation in sea-surface temperature, precipitation, and sea-level pressure in the equatorial Pacific known as the El Niño - Southern Oscillation (ENSO) is an important mode of interannual variability in global climate. A collapse of the tropical Pacific onto a state resembling a so-called 'permanent El Niño', with a preferentially warmed eastern equatorial Pacific, flatter thermocline, and reduced interannual variability, in a warmer world is predicted by prevailing ENSO theory. If correct, future warming will be accompanied by a shift toward persistent conditions resembling El Niño years today, with major implications for global hydrological cycles and consequent impacts on socioeconomic and ecological systems. However, much uncertainty remains about how interannual variability will be affected. Here, we present multi-annual records of climate derived from growth increment widths in fossil bivalves and co-occurring driftwood from the Antarctic peninsula that demonstrate significant variability in the quasi-biennial and 3-6 year bands consistent with ENSO, despite early Eocene (~50 Mya) greenhouse conditions with global average temperature -10 degrees higher than today. A coupled climate model suggests an ENSO signal and teleconnections to this region during the Eocene, much like today. The presence of ENSO variation during this markedly warmer interval argues for the persistence of robust interannual variability in our future greenhouse world