T129 Advances in Non-Traditional Stable Isotope Measurements and Utility as Proxies in Modern and Paleo-Settings II

Monday, 16 October 2023
1:30 PM - 5:30 PM
335

MARINE STABLE POTASSIUM ISOTOPE MASS BALANCE AND ITS IMPLICATIONS FOR THE MODERN AND PAST CARBON CYCLE

Paper number: T129-8
Program: Technical Sessions
Time: 3:05 PM – 3:20 PM

Authors

Abstract

Authigenic clay formation has been proposed as an essential process that counterbalances the effect of silicate weathering, providing a critical mechanism to stabilize atmospheric pCO2 and ocean chemistry over geological timescales. Although evidence for local authigenic clay formation is emerging, constraining the global significance of this process remains challenging. Stable K isotopes (41K/39K or δ41K), enabled by recent analytical advances, offer a novel tool to study the poorly-constrained aspect of the silicate cycle because of the primary occurrence of K in silicates, including clays.

An outstanding but unexplained feature of available δ41K data is that seawater is ~0.6‰ higher than the bulk silicate Earth (BSE) [1]. Here we present our recent work intended to better constrain the K isotope mass balance for the modern ocean, with a particular focus on hydrothermal systems and marine authigenic glauconites. Through analysis of natural hydrothermal fluids [2] and controlled laboratory experiments, we discovered (1) phase separation can produce resolvable K isotope fractionation of ~0.1‰, but this fractionation is too small to explain δ41K variability in natural fluids; (2) low-temperature hydrothermal alteration of the oceanic crust enriches heavy K isotopes in fluids, but this isotopic effect is erased by high-temperature alteration that fractionates K isotopes to an opposite direction. Together with riverine δ41K data in literature [3, 4], our results indicate that neither rivers nor hydrothermal circulation can fully explain the high δ41K value of seawater. We further analyzed recent marine authigenic glauconites extracted from two sediment cores–one from the equatorial Atlantic (ODP 959) and the other from the NE Pacific Oregon margin. We discovered that authigenic glauconites have δ41K significantly lower (i.e., ~1‰) than that of seawater, indicating that authigenic clay formation is critical in producing the high δ41K signature of modern seawater. We will provide an updated marine K isotope mass balance based on our results. Interestingly, glauconites analyzed from the two sites of distinct depositional settings showed consistent δ41K, implying that marine authigenic glauconites can be a valuable archive for ancient seawater δ41K.

[1] Wang et al., Geochemistry, 2021; [2] Zheng et al., EPSL, 2022; [3] Li et al., PNAS, 2019; [4] Wang et al., GCA, 2021.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-392271

© Copyright 2023 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.