Carbon Isotopic Signature and Organic Matter Composition of Cenomanian High-Latitude Paleosols of Southern Patagonia

Augusto Nicolás Varela,Ulrich Heimhofer,Sabrina Lizzoli,Thierry Adatte,María Sol Raigemborn,Patricio Emmanuel Santamarina

doi:10.3390/geosciences11090378

Abstract

The Cenomanian Mata Amarilla Formation (MAF) in southern Patagonia (~55° S paleolatitude, Austral-Magallanes Basin, Argentina) is composed mainly of stacked fluvial deposits with intercalated paleosols, which document Cenomanian environments at high-paleolatitudes in the Southern Hemisphere. We performed a multiproxy study of the paleosols and sediments of the MAF in order to (1) understand the composition of the soil- and sediment-derived organic matter (OM), (2) apply carbon isotope stratigraphy as a tool to correlate patterns obtained from the MAF with existing marine and non-marine δ13Corg records worldwide, and (3) investigate the relationship between variations in spore-pollen assemblages of the MAF and the climatic conditions prevailing in the Cenomanian Southern Hemisphere. An integrated dataset was generated, including total organic carbon content, Rock-Eval pyrolysis data, stable isotope (δ13Corg) composition, and palynological data, combined with published paleosol-derived mean annual temperatures and mean annual precipitations. The results indicated that the OM preserved in the MAF paleosols allowed its use as a chemostratigraphic tool. The MAF δ13Corg curve showed the rather stable pattern characteristic for the Early to Late Cenomanian interval. The absence of the major positive carbon isotope excursion associated with oceanic anoxic event 2 provided an upper limit for the stratigraphic range of the MAF. The palynological data suggested the development of fern prairies during warmer and moister periods at the expense of the background gymnosperm-dominated forests. Overall, the multiproxy record provided new insights into the long-term environmental conditions during the Cenomanian in the high latitudes of the Southern Hemisphere.

Highlights

All organic geochemical analyses, paleosol-derived geochemical climofunctions (MAT and mean annual precipitation (MAP); [8]) and palynological investigations were carried out using the same bulk rock sample in order to provide a consistent data-set and to eliminate potential stratigraphic errors
Lower mean total organic carbon (TOC) contents were recorded in Inceptisols samples, while highest mean TOC contents were observed in Histosols and lacustrine deposits (Figure 3A)
The carbon isotope record from the Mata Amarilla Formation (MAF) was largely independent of paleosol type and depositional environment, demonstrating that the δ13 C signature of soil-derived

Summary

Introduction

Warm temperatures and the acceleration of the hydrological cycle [11,12,13,14,15] led to increased primary productivity, causing oceanic anoxia [16,17], expressed in the widespread deposition of organic carbon-rich strata representing the so-called oceanic anoxic events (OAEs; [18,19,20]). Cretaceous OAEs are typically associated with relatively long-lasting positive and/or negative carbon isotope excursions (CIEs) in the range of several per mil (‰), which are interpreted to record shifts in the global carbon cycle [6,19,21,22].

Methods

Results

Discussion

Conclusion