Gradients in seasonality and seawater oxygen isotopic composition along the early Permian Gondwanan coast, SE Australia

Abstract

Oxygen isotope compositions of marine carbonates are commonly employed for understanding ancient temperatures, but this approach is complicated in the very distant past due to uncertainties about the effects of diagenesis and the isotopic composition of seawater, both locally and globally. Microsampled accretionary calcite from two species of the fossil bivalve Eurydesma Sowerby and Morris 1845 collected from sediments of Cisuralian age in high latitude marine sediments along the SE coast of Australia records cyclic seasonal fluctuations in shell δ 18 O values during growth, demonstrating the primary nature of the isotope signal and thus allowing investigation of early Permian seawater isotopic composition and water temperature in the high southern latitudes. The mean and seasonal range of δ 18 O carb decreases poleward across about 10 • of paleolatitude (∼67 • S-77 • S). The presence of co-occurring dropstones and stratigraphically associated glendonites constrains winter temperatures across the region to near-freezing, thus permitting calculation of realistic estimates of water composition and summer temperatures.Summer δ 18 O carb values indicate water temperatures between 5 • C and 12 • C, with warmer values at lower latitudes. The decrease in both mean sea surface temperature and seasonal amplitude with increasing latitude on the Gondwanan coast is much like that observed along high-latitude coastlines today. Calculated δ 18 O water decreases toward the pole, likely associated with an increasing contribution of isotopically light fresh water derived from summer snow-melt. The gradient in δ 18 O water is similar to that documented over a similar span of latitude on the modern SE Greenland coast. We infer the presence of a north-flowing coastal current of cold, O 18 -depleted water that entrains progressively greater amounts of more typical seawater as it moves away from the pole. δ 18 O values in SE Australia, however, are about 3h lower than those off Greenland, suggesting comparatively lower salinity water or more O 18 -depleted glacial ice/runoff in the Permian Gondwanan high latitudes, perhaps augmented by more depleted (negative) global average seawater. Conditions in southeastern Australia during the largest of the Permian deglaciations were warmer than present-day Antarctica at similar latitudes, but may approximate those of early-mid Miocene Antarctica, with frozen winters but summers closer to 10 • C.

High-latitude settings promote extreme longevity in fossil marine bivalves

“…The same is true of bivalves from the fossil record. Buick and Ivany (2004) and Ivany et al (2008) documented annual growth bands in Cucullaea raea and Eurhomaela antarctica (now Retrotapes antarcticus , Alvarez et al 2014) from the Eocene of Seymour Island, Antarctica, and Beard et al (2015) and Ivany and Runnegar (2010) did the same with early Permian bivalves from SE Australia. In the past and today, the polar regions are characterized by extreme seasonality in light regime and hence phytoplankton production.…”

Improving shallow‐water carbonate chemostratigraphy by means of rudist bivalve sclerochemistry

“…In contrast, sclerochronological oxygen‐isotope profiles provide evidence for cyclic (seasonal) oxygen‐isotope variations related either to temperature or salinity (see supporting information), an observation that underlines the excellent preservation state of the rudist shells [cf. Beard et al ., ]. As expected, rudists provide sclerochronological carbon‐isotope (δ 13 C sclero ) variabilities in the order of up to 1.5‰ (mean: 1.1‰; standard deviation: 0.3‰; Figure ).…”

Fossil bivalves and the sclerochronological reawakening

“…Scale bars for A-C 0.5 cm; scale bars for D and E, 1 cm. Annual nature of banding in these taxa is documented in Buick and Ivany (2004); Ivany andRunnegar (2007, 2010); Ivany et al (2008); Beard et al (2015); Moss et al (2017); Judd et al (2019). represent a "faster" approach to the asymptotic size (L ∞ ), and body size positively correlates with reproductive potential in bivalves (e.g., Peterson 1986;Nakaoka 1994).…”