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.

Reconstructions of aeolian dust flux to West African margin sediments can be used to explore changing atmospheric circulation and hydroclimate over North Africa on millennial to orbital timescales. Here, we extend West African margin dust flux records back to 35 ka in a transect of core sites from 19°N to 27°N, and back to 67 ka at Ocean Drilling Program (ODP) Hole 658C, in order to explore the interplay of orbital and high-latitude forcings on North African climate and make quantitative estimates of dust flux during the core of the Last Glacial Maximum (LGM). The ODP 658C record shows a “Green Sahara” interval from 60 to 50 ka during a time of high Northern Hemisphere summer insolation, with dust fluxes similar to levels during the early Holocene African Humid Period, and an abrupt peak in flux during Heinrich event 5a (H5a). Dust fluxes increase from 60 to 35 ka while the high-latitude Northern Hemisphere cools, with peaks in dust flux associated with North Atlantic cool events. From 35 ka through the LGM dust deposition decreases in all cores, and little response is observed to low-latitude insolation changes. Dust fluxes at sites north of 20°N were near late Holocene levels during the LGM time slice, suggesting a more muted LGM response than observed in mid-latitude dust sources. Records along the northwest African margin suggest important differences in wind responses during different stadials, with maximum dust flux anomalies centered south of 20°N during H1 and north of 20°N during the Younger Dryas.

Creeping faults are typically not associated with large earthquakes. However, new K/Ar dating and biomarker maturity data on the San Andreas Fault Observatory at Depth (SAFOD) present evidence that large paleoearthquakes have occurred in the creeping section of the San Andreas Fault, California. K/Ar ages of bulk samples with evidence of coseismic heating range from 3.3 to 15.8 Ma, and argon diffusion experiments suggest that these ages are only partially reset and the actual event ages may be even younger. Thus, questions remain as to how we can refine such dates to reveal the precise age and location of these earthquakes. To refine the ages and more accurately assess seismic hazard, we date size separates of eight samples from different sections of the SAFOD core. Following Stokes’ Law, we split each sample into five size fractions using hydrodynamic settling: <0.2, 0.2-0.5, 0.5-0.8, 0.8-1.4, and 1.4-2 micrometers. The finest size fractions contain the most authigenic illite, which form during fault slip. We determined chemical composition and separated illite polytypes using x-ray diffraction, and also measured K/Ar ages on each sample. Preliminary results from two scaly black fault rock samples, previously shown to have hosted earthquakes, (3,193.69 m and 3,193.96 m along the core) support that the finest size fractions contain the greatest ratio of authigenic illite. With a York regression between age and detrital illite abundance, we place the authigenic illite ages at 1.08 ± 2.40 Ma and 0.88 ± 5.08 Ma for these two samples, and observe that the detrital illite matches the late Cretaceous age for the country rock. This new age estimate for the authigenic illite means that large earthquakes must have propagated into the creeping section within the last million years. Not only is it significantly younger than the bulk sample age, it is recent enough that translation of faulted material from the locked southern San Andreas fault into the creeping section cannot explain the record. Moving forward, we will expand our procedure to include isotope dilution for measuring K concentration and analyze the other samples previously measured for biomarker maturity and bulk K/Ar age. Resulting insights into the fault rock composition and the timing of past earthquakes will be crucial in assessing the region’s seismic hazard.