No effect of thermal maturity on the Mo, U, Cd, and Zn isotope compositions of Lower Jurassic organic-rich sediments

Alexander J Dickson,Erdem Idiz,Simon W Poulton,Melissa J Murphy,Micha Ruhl,Stephen P Hesselbo,John N Hooker,Sander H.J.M Van Den Boorn,Don Porcelli,Hugh C Jenkyns,Ricardo Celestino

doi:10.1130/g49724.1

Abstract

AbstractThe isotope ratios of redox-sensitive metals in organic-rich rocks are critical tools for quantifying the timing and severity of deoxygenation and nutrient cycling in Earth's past. The resilience of isotopic data to thermal alteration of the host sediments over millions of years of burial is, however, largely unknown. We present molybdenum, uranium, cadmium, and zinc stable-isotope data from two stratigraphic successions of the same Lower Jurassic Posidonienschiefer unit in the Lower Saxony Basin of northern Germany that were affected by different burial histories. We show that thermal maturity had no effect on the isotopic compositions of these elements but does appear to have increased their concentrations in the rock. The data corroborate the results of laboratory-based maturation studies and provide constraints on the Mo, U, Cd, and Zn isotopic compositions of ca. 182 Ma seawater in the Lower Saxony Basin.

Highlights

Isotopic measurements of metallic elements are widely employed as a means to elucidate the redox history of the oceans
These characteristics of the Posidonienschiefer deposits in the Lower Saxony Basin (LSB) offer the ideal opportunity to test whether thermal maturation can alter the isotopic composition and concentration of metals contained within organic-rich sedimentary rocks
Slight differences in isotopic compositions stratigraphically above and below the Posidonienschiefer unit are probably related to the lower nondetrital concentrations of the elements in these deposits

Summary

Introduction

Isotopic measurements of metallic elements are widely employed as a means to elucidate the redox history of the oceans. The organic matter contained within the Posidonienschiefer unit has been exposed to vastly different degrees of heating over horizontal distances of tens of kilometers. These characteristics of the Posidonienschiefer deposits in the LSB offer the ideal opportunity to test whether thermal maturation can alter the isotopic composition and concentration of metals contained within organic-rich sedimentary rocks

Methods

Results

Conclusion