Paleoceanography of the Late Cretaceous northwestern Tethys Ocean: Seasonal upwelling or steady thermocline?

Abstract

In this study we attempted to assess whether seasonal upwelling or a steady thermocline persisted at the western margin of the Tethys Ocean during the late Turonian-early Coniacian interval. For this scope, we employed novel and published stable oxygen isotope (δ 18 O) data of various organisms (bivalves, bivalves, brachiopods, fish and belemnites). New seasonally resolved temperature estimates were based on the δ 18 O record of sequentially sampled inoceramid (Inoceramus sp.) and rudist (Hippurites resectus) shells from the Scaglia Rossa and Gosau deposits of northern Italy and western Austria, respectively. Diagenetic screening was performed using reflected light, cathodoluminescence (CL), scanning electron microscopy (SEM) and stable isotope analysis. Originally preserved δ 13 C and δ 18 O values were used to characterize the lifestyle of the bivalves and detect vital effects that could have biased oxygen isotope-based temperature reconstructions. Inoceramid δ 18 O values provide-for the first time-information on temperatures of Tethyan benthic waters, which were, on average, 14.4 ± 0.6˚C and fluctuated seasonally within a range of less than 2˚C. Such a thermal regime is in line with the temperatures postulated for late Turonian boreal water masses and support the existence of a cold water supply from the North Atlantic to the Tethyan bottom. Bottom cooling, however, did not affect the shallow water environment. In fact, the rudist-based temperature estimates for shallow water environment revealed a mean annual range of 11˚C, between 24 and 35˚C (assuming a seasonally constant δ18O w = 1.0 ‰), which are among the warmest temperatures recorded over the entire Late Cretaceous. Our findings, thus, suggest a strong thermal and food web decoupling between the two environments. The absence of a seasonal vertical homogenization of different water bodies suggests the existence of a steady thermocline and, therefore, contrasts with the presence of an active coastal upwelling in the region as hypothesized by previous authors.

Early Cretaceous sea surface temperature evolution in subtropical shallow seas

“…One explanation for the in general high SST seasonalities would be that recorded seasonal δ 18 O patterns also reflect local salinity-controlled δ 18 O SW changes. In a previous sclerochronological study on a Late Cretaceous subtropical elevator rudist 58 , the authors pointed out that the recorded seasonal amplitude of 11 °C maybe overestimated by up to 7.5 °C due to a seasonal δ 18 Osw fluctuation of up to 1.5‰. Considering the paleogeographic position of the here studied Northern Tethyan and proto-North Atlantic rudist-bearing sections in the subtropical arid belt, significant seasonal δ 18 O SW variations (> 1‰) are unlikely 27 .…”

An Analysis of the Earth’s Energy Budget

“…Variations in water distribution in the forms of either reflective ice and/or cloud; or absorbing surface water areal variations by either short term sea-ice distribution or long-term geologic ocean distribution (e.g. The Cretaceous Tethys Ocean [12]) is the primary route to change planetary albedo.…”

Astronomically controlled deep‐sea life in the Late Cretaceous reconstructed from ultra‐high‐resolution inoceramid shell archives

“…Moreover, indistinct internal laminae structure and micro-growth increments visibility in thin sections cause problems with estimating the ontogenetic age of analysed material by visual counting. Typically, the pattern of bivalve shell growth increments is explicitly observed in modern (Schöne & Giere, 2005) as well as fossil bivalves (Walliser & Schöne, 2020). The growth is marked by alternating light and dark lamina created, respectively, during fast and slow biomineralization, where the width of increments is controlled by environmental conditions (Schöne, 2008).…”

“…Their wide ecological tolerance and highly adaptive strategy of life allowed them to inhabit environments ranging from highly oxygenated shallow‐marine to oxygen‐deficient deep‐marine ones (Kauffman & Harries, 1996). Hence, they can not only be used as an excellent stratigraphic tool (Walaszczyk, 2004) but also have high analytical capabilities in reconstructions of paleoenvironmental proxies (Walliser & Schöne, 2020). The present study reveals rhythmic patterns recorded based on high‐resolution geochemical data derived from the Upper Cretaceous inoceramid bivalve shells of the species I .…”