Iron (Fe) availability impacts marine primary productivity, influencing the efficiency of the biological pump. Stable Fe isotope analysis has emerged as a tool to understand how Fe is sourced and cycled in the water column. However, distinguishing the major Fe sources in sediment records based on their Fe isotope compositions alone is challenging as sources can exhibit overlapping values. To address this issue, we studied three pelagic clay sequences spanning the past 90 million years. First, we used osmium isotope chronostratigraphy to date the sequences. Second, we analyzed leachates of the sediments for their multi-element concentration and Fe isotope compositions. Third, we used statistical modeling to reveal five principal Fe sources to the sites—dust, distal background, two hydrothermal fluids, and a magnesium-rich volcanic ash—which share similar depositional histories. Initially, hydrothermal inputs dominated Fe deposition, shifting to increased contributions from other sources as the sites migrated away from their respective mid-ocean ridges. Notably, between 66–40 Ma, distal background sources became significant, before the sites shifted to increasing dust dominance around 30 million years ago. Our analysis suggests that the modern South Pacific is the most dust-dominated it has been in ≈90 million years, which is significant considering the low rates of dust deposition to this region. Our approach establishes a framework combining isotope measurements, geochemical analysis, and statistical modeling to explore the history of Fe delivery to the ocean. This approach holds potential for understanding the interplay between Fe delivery, the biological pump, and Earth’s climate across geological timescales.

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.

What controls the distribution of barium (Ba) in the oceans? Answers to this question have been sought since early studies revealed relationships between particulate Ba (pBa) and POC and dissolved Ba (dBa) and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. Herein, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ~50% of the dBa inventory (upper 500 m of the Arctic water column) is not supplied by conservatively advected inputs. Calculated areal dBa fluxes are up to 10 µmol m-2 d-1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. Surprisingly, the Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin-bay derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a terrigenous source (e.g., submarine groundwater discharge or fluvial particles) is an important contributor