Continental Climatic and Weathering Response to the Eocene-Oligocene Transition

Abstract

Paleoclimatic reconstructions of the Eocene-Oligocene transition indicate significant spatial heterogeneity in both the marine and the terrestrial responses to the formation of ice sheets on Antarctica. Marine isotopic records indicate that both cooling and ice volume effects contributed to a shift of approximately ϩ1.1‰ in benthic and planktonic foraminiferal d 18 O. Because terrestrial records generally are of lower temporal resolution, deconvolving temperature from ice volume effects has been more challenging. New results based on paleosols in a well-dated terrestrial sequence in the Ebro Basin (Spain) bridge this gap by providing a new high-resolution record of paleoclimate and paleoweathering. Between 35 and 33 Ma, the reconstructed mean annual precipitation was unchanged, and mean annual temperatures ranged between ∼8Њ and 14ЊC, with identical means of ∼11ЊC in both the Eocene and the Oligocene, indicating that paleotemperature was steady in the Ebro Basin. At the same time, a drop in chemical weathering of 130% that was accompanied by a roughly 20% drop in sedimentation rate was observed coincident with declining atmospheric CO 2 levels. Prior to and following the Eocene-Oligocene transition, pedogenic carbonate d 18 O values moved in concert with weathering, but this connection was broken during the Eocene-Oligocene transition itself, suggesting a significant hydrological cycle reorganization at this time. Thus, while the changes in chemical weathering cannot be due to precipitation or temperature changes, the decreased chemical weathering was likely due to falling atmospheric CO 2 levels. This suggests that long-term records of paleosol weathering intensity provide a new proxy for relative atmospheric CO 2 changes.

Quasi-static Eocene–Oligocene climate in Patagonia promotes slow faunal evolution and mid-Cenozoic global cooling

“…Omitting a few outliers at low  18 O and high  13 C, which may reflect nodules that formed closer to paleosurfaces, average values are  13 C = -7.3±0.4‰ (V-PDB; 2) and  18 O = 21.2±1.0‰ (V-SMOW; 2). These are some of the first isotope data from definitive pedogenic carbonates collected proximal to the EOT, and  13 C values are well within the range reported globally for Eocene and Oligocene paleosols (Ekart et al, 1999;Sheldon and Tabor, 2009;Sheldon et al, 2012;Srivastava et al, 2013). In combination with published tooth enamel  18 O and  13 C values, p CO2 and paleotemperature may be estimated.…”

Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition

“…While the large sustained d 18 O shift recorded in the Hampshire Basin is not recorded in European endorheic basin records (Maians, Spain; Fig. 4D), similar reorganization of the hydrologic cycle was previously identified from high-magnitude variability in paleosol carbonate d 18 O values (Sheldon et al, 2012). Because the poles cooled more rapidly during the Eocene-Oligocene transition than lower-latitude sites (e.g., Liu et al, 2009), the changing ocean and air temperature gradients may have changed the moisture source amounts more at higher latitudes as well.…”

“…Zanazzi et al, 2007;Zanazzi and Kohn, 2008), which instead often show no discernible shift in d 18 O (e.g., Sheldon et al, 2012) or a delayed shift relative to marine records (Zanazzi et al, 2007). In addition, it can be difficult to link marine and terrestrial records of the Eocene-Oligocene transition for two reasons: (1) Continental successions are most often preserved in endorheic basins, far from marine incursions that would make direct age comparison possible, and (2) oxygen isotope records from continental interiors can be complicated by a variety of non-temperaturerelated factors (e.g., changing circulation patterns, orographic effects) or may respond to local, rather than global hydrologic cycle drivers (e.g., Sheldon et al, 2012). Thus, sections in continental strata that span the Eocene-Oligo cene transition and that are well calibrated to the geomagnetic polarity time scale and to marine geochronology are rare.…”

Contrasting geochemical signatures on land from the Middle and Late Permian extinction events

“…The ∆W proxy was recently applied by Sheldon et al . () to look at weathering changes across the Eocene–Oligocene transition, where additional a significant reduction was demonstrated in chemical weathering that occurred concomitantly with a drop in atmospheric pCO 2 and by Retallack et al . () who demonstrated significantly enhanced ∆W values in concert with increasing atmospheric pCO 2 .…”