Abstract
Reconstructions of past oceanic redox conditions, based on the measurement of metal distributions and their stable-isotope ratios in marine sediments, have now become more commonplace, providing new constraints on past environmental change. Almost all of these records, however, are based on organic-rich black-shale and carbonate sedimentary archives, which have formed primarily in low-latitude regions. This limitation leads to incomplete geographic coverage that complicates the global-scale interpretation of the results. In this study, the potential of oceanic ‘red beds’ and grey shales as new archives of past environmental conditions are explored to complement the datasets derived from other lithologies and extend reconstruction efforts to mid- to high-latitude regions. Our records originate from open-marine red and grey shales from two sedimentary sections spanning the Late Cretaceous Oceanic Anoxic Event 2 (OAE 2) in New Zealand, formerly deposited at high southern latitudes in the palaeo-Pacific Ocean at c. 94 Ma and where contemporaneous black-shale and/or carbonate successions do not exist. Access to the seawater-derived, authigenic fraction of the bulk sediment is essential for reconstructing past marine redox state at these mid- to high-latitude locations. Therefore, a series of leaching experiments using reagents of progressively increasing concentration were conducted to determine the best method for isolating the authigenic Fe- and Mn-(oxyhydr)oxide fraction from the red and grey shale lithologies. Chemostratigraphic records for the concentrations of a suite of redox-sensitive metals (Fe, Mn, Co, Ni, Mo, Cr, V and U), as well as the U-isotope palaeo-redox tracer (238U/235U; reformulated as δ238U), were obtained. These records imply that leaching with 6 M HCl at room temperature is the best of the tested methods for the selective extraction of the Fe- and Mn-(oxyhydr)oxide phase, whilst minimising the contribution from older inherited detrital material that potentially could confound the interpretation of the chemostratigraphic records. This method offers potential for reliably constraining the authigenic distributions of redox-sensitive elements in red and grey shales, provided Fe- and Mn-(oxyhydr)oxides are predominantly of authigenic origin and constitute at least 5 % of the mineralogy. By contrast, for the investigation of the U-isotope system, a larger Fe- and Mn-(oxyhydr)oxide proportion in the sediment for an ocean–atmosphere perturbation event recording more severe changes in ocean redox conditions and bigger U-isotope shifts than observed for OAE 2 are both necessary to obtain reliable results.
•The potential of red and grey shales as palaeo-environmental archives is explored.•Leaching experiments identified a method to isolate the (oxyhydr)oxide fraction.•Shows potential for trace metal studies in sediments with >5 % (oxyhydr)oxides.•The application of U isotopes requires further research.