Bioactive trace metals and their isotopes as paleoproductivity proxies:
An assessment using GEOTRACES-era data
Abstract
Phytoplankton productivity and export sequester climatically significant
quantities of atmospheric carbon dioxide as particulate organic carbon
through a suite of processes termed the biological pump. How the
biological pump operated in the past is therefore important for
understanding past atmospheric carbon dioxide concentrations and Earth’s
climate history. However, reconstructing the history of the biological
pump requires proxies. Due to their intimate association with biological
processes, several bioactive trace metals and their isotopes are
potential proxies for past phytoplankton productivity, including: iron,
zinc, copper, cadmium, molybdenum, barium, nickel, chromium, and silver.
Here we review the oceanic distributions, driving processes, and
depositional archives for these nine metals and their isotopes based on
GEOTRACES-era datasets. We offer an assessment of the overall maturity
of each isotope system to serve as a proxy for diagnosing aspects of
past ocean productivity and identify priorities for future research.
This assessment reveals that cadmium, barium, nickel, and chromium
isotopes offer the most promise as tracers of paleoproductivity, whereas
iron, zinc, copper, and molybdenum do not. Too little is known about
silver to make a confident determination. Intriguingly, the elements
that are least sensitive to productivity may be used to trace other
aspects of ocean chemistry, such as nutrient sources, particle
scavenging, organic complexation, and ocean redox state. These
complementary sensitivities suggest new opportunities for combining
perspectives from multiple proxies that will ultimately enable painting
a more complete picture of marine paleoproductivity, biogeochemical
cycles, and Earth’s climate history.