Effects of redox variability and early diagenesis on marine sedimentary
Volcanism is the dominant natural source of mercury (Hg) to the
atmosphere, biosphere, ocean and sediments. In recent years, sedimentary
Hg contents have emerged as a tool to reconstruct volcanic activity, and
particularly activity of (subaerially emplaced) large igneous provinces
(LIP) in geological deep time. More specifically, Hg has shown potential
as a useful proxy to illuminate the previously elusive impact of such
large-scale volcanism on marine and terrestrial paleo-environments.
While Hg is now widely applied as volcanism tracer, non-volcanic factors
controlling sedimentary Hg content are generally not well constrained.
Part of this uncertainty stems from our inability to directly observe a
natural unperturbed “steady-state” environment as a baseline, as the
modern Hg cycle is heavily influenced by anthropogenic activity. Here we
focus on the effects of ambient redox conditions in the water column and
shallow sediments (early diagenesis), quantify their influence on the
geological Hg record and thereby constrain their potential impact on the
use of Hg as a proxy for deep-time volcanic activity. Constraining these
factors is of critical importance for the application of Hg as a proxy.
Many periods in the geological past for which records have been
generated, such as the Mesozoic Oceanic Anoxic Events, are marked by a
variety of high-amplitude environmental perturbations, including
widespread deoxygenation and deposition of organic-rich sediments. We
estimate the impact of redox changes and early diagenesis on the
geological Hg record using a suite of (sub)recent–Pleistocene and Upper
Cretaceous sediments representing oxic to euxinic marine conditions. Our
sample set includes a transect through an oxygen minimum zone and cores
that record transient shifts in oxygenation state, as well as
post-depositional effects – all unrelated to volcanism, to the best of
our knowledge. We find substantial alterations to the Hg record and the
records of organic carbon and total sulfur, which are typically assumed
to be the most common carrier phases of Hg in marine sediments.
Moreover, these biases can lead to signal-alterations on a par with
those interpreted to result from volcanic activity. Geochemical
modifications are ubiquitous and their potential magnitude implies that
the factors leading to biases in the geological record warrant careful
consideration before interpretation. Factors of particular concern to
proxy application are (1) the disproportionate loss of organic carbon
and sulfur compounds relative to Hg during oxidation that strongly
modulates normalized Hg records, (2) the evasion of Hg in anoxic and
mildly euxinic sediments and (3) sharp focusing of Hg during
post-depositional oxidation of organic matter.