Abstract
Abstract. "Hyperthermals" are past intervals of geologically rapid global warming that provide the opportunity to study the effects of climate change on existing faunas over thousands of years. A series of hyperthermals is known from the early Eocene (~ 56–54 million years ago), including the Paleocene–Eocene Thermal Maximum (PETM) and two subsequent hyperthermals (Eocene Thermal Maximum 2 – ETM2 – and H2). The later hyperthermals occurred during warming that resulted in the Early Eocene Climatic Optimum (EECO), the hottest sustained period of the Cenozoic. The PETM has been comprehensively studied in marine and terrestrial settings, but the terrestrial biotic effects of ETM2 and H2 are relatively unknown. Two carbon isotope excursions (CIEs) have been described in the northern part of the Bighorn Basin, WY, USA, and related to ETM2 and H2. An ~ 80 m thick zone of stratigraphic section in the extraordinarily dense, well-studied terrestrial mammal fossil record along the Fifteenmile Creek (FC) in the south–central part of the basin spans the levels at which the CIEs occur in the northern Bighorn Basin. High-resolution, multiparameter paleoecological analysis of this part of the FC section reveals two episodes of significant faunal change – faunal events B-1 and B-2 – characterized by significant peaks in species diversity and turnover and changes in abundance and relative body size. Faunal events B-1 and B-2 are hypothesized to be related to the CIEs in the northern part of the basin and hence to the climatic and environmental change of ETM2 and H2. In contrast with the PETM, change at faunal events B-1 and B-2 is less extreme, is not driven by immigration and involves a proliferation of body sizes, although abundance shifts tend to favor smaller congeners. The response at faunal events B-1 and B-2 is distinctive in its high proportion of species losses, potentially related to heightened species vulnerability in response to changes already underway in the lead-up to the EECO. Faunal response at faunal events B-1 and B-2 is also distinctive in that it shows high proportions of beta richness, suggestive of increased geographic dispersal related to transient increases in habitat (floral) complexity and/or precipitation or seasonality of precipitation.
Highlights
The late Paleocene and early Eocene was an interval of global warming and massive inputs of carbon to the ocean and atmosphere (Zachos et al, 2008)
Three overlapping peaks occur in both sets of parameters (370– 394 m, 405–417 m and 435–448 m), significantly refining previous work at a coarser resolution (20 m thick intervals), in which a single, 40 m thick interval (370–410 m) of biotic change was identified in this part of the Fifteenmile Creek (FC) section (Chew, 2009a)
Faunal events B-1 and B-2 are set in the context of pronounced climatic, environmental and faunal change attributed to warming in the run-up to the Eocene Climatic Optimum (EECO) (Chew, 2009a; Woodburne et al, 2009)
Summary
The late Paleocene and early Eocene (ca. 58–51 Myr) was an interval of global warming and massive inputs of carbon to the ocean and atmosphere (Zachos et al, 2008). The Earth’s surface warmed from the late Paleocene through the Early Eocene, culminating in the Early Eocene Climatic Optimum (EECO), the hottest sustained period of the Cenozoic (∼ 53–50 Myr; Zachos et al, 2001, 2008) Superimposed on this long-term change were several “hyperthermals”, short-term (< 100 kyr) warming events (Cramer et al, 2003; Lourens et al, 2005). The hyperthermals are marked by large decreases in the δ13C composition of carbon-bearing phases in sedimentary strata, which are referred to as carbon isotope excursions (CIEs), and carbonate dissolution in deep-sea sediment The latter suggests that the hyperthermals were related to massive inputs of reduced carbon to the ocean and atmosphere (Zachos et al, 2005). A comparison of multiple excursions in diverse carbon isotope records from the PETM indicate
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