MARINE STABLE POTASSIUM ISOTOPE MASS BALANCE AND ITS IMPLICATIONS FOR THE MODERN AND PAST CARBON CYCLE (Invited Presentation)

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.