Release from biogenic particles, benthic fluxes, and deep water
circulation control Cr and δ 53 Cr distributions in the ocean interior
Abstract
Chromium (Cr) has shown promise as a paleoceanographic proxy due to the
redox-driven control of dissolved Cr concentrations ([Cr]) and
stable isotope composition (δ53Cr). However, substantial uncertainties
in the biogeochemical Cr cycle have limited its paleoproxy application
to date. To improve the mechanistic understanding of Cr cycling in the
modern ocean and strengthen its potential proxy applications, we present
new data from regeneration incubations, bottom and sediment pore waters,
and a compilation of intermediate and deep water data. While Cr removal
and biological export from the surface ocean is associated with organic
carbon export, the deep water release of dissolved Cr from sinking
particles is not directly dependent on organic carbon respiration, as
indicated by differing trends between Cr, oxygen utilization and the
regeneration of organic-associated macronutrients (e.g. N, P). Pore
water and bottom water data demonstrate that benthic Cr fluxes are
locally important and may be significant globally. The pore water
dissolved Cr flux at our CaCO3-rich site is likely driven by the
re-release of Cr scavenged from the water column by sinking particles,
with minor contributions from lithogenic phases. We argue this is
consistent with the highest open ocean [Cr] to date being found in
the water column below oxygen minimum zones, likely reflecting the
release of scavenged Cr in deep waters or surface sediments. Chromium
released from suspended particles and surface sediments follows the
global δ53Cr–[Cr] array, supporting the proposed role of biological
export and regeneration in shaping global Cr and δ53Cr distributions.
Global intermediate and deep water [Cr], δ53Cr and Cr:macronutrient
relationships are thus shaped by a synergy of circulation patterns,
water mass mixing, a deep Cr regeneration cycle, and benthic Cr sources.
A biogenic control on global Cr distributions indicates that sedimentary
Cr records may reflect biogenic as well as O2-dependent processes, while
more research is needed to assess sediment Cr record fidelity based on
an active diagenetic cycle.