Manganese (Mn) is an essential element for photosynthetic life, yet concentrations in Southern Ocean open waters are very low, resulting from biological uptake along with limited external inputs. At southern latitudes, waters overlying the Antarctic shelf are expected to have much higher Mn concentrations due to their proximity to external sources such as sediment and sea ice. In this study, we investigated the potential export of Mn-rich Antarctic shelf waters toward depleted open Southern Ocean waters. Our results showed that while high Mn concentrations were observed over the shelf, strong biological uptake decreased dissolved Mn concentrations in surface waters north of the Southern Antarctic Circumpolar Current Front (< 0.1 nM), limiting export of shelf Mn to the open Southern Ocean. Conversely, in bottom waters, mixing between Mn-rich Antarctic Bottom Waters and Mn-depleted Low Circumpolar Deep Waters combined with scavenging processes led to a decrease in dissolved Mn concentrations with distance from the coast. Subsurface dissolved Mn maxima represented a potential reservoir for surface waters (0.3 – 0.6 nM). However, these high subsurface values decreased with distance from the coast, suggesting these features may result from external sources near the shelf in addition to particle remineralization. Overall, these results imply that the lower-than-expected lateral export of trace metal-enriched waters contributes to the extremely low (< 0.1 nM) and potentially co-limiting Mn concentrations previously reported further north in this Southern Ocean region.

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.

Phytoplankton indirectly influence the climate, through their role in the ocean biological carbon pump. Hence factors limiting phytoplankton growth directly impact the strength of the biological carbon pump and consequently climate. In the Southern Ocean, the subantarctic zone represents an important carbon sink, yet variables limiting phytoplankton growth are not fully constrained. Co-limitation by iron (Fe) and manganese (Mn) has recently been observed in the coastal and offshore Southern Ocean, but very few studies have focused on the subantarctic zone. In addition, no study has investigated the seasonal variability of Mn (co-)limitation of phytoplankton growth in the Southern Ocean. Using three shipboard bioassay experiments, we evaluated the seasonality of Fe and Mn co-limitation of subantarctic phytoplankton growth, south of Tasmania. We observed a strong seasonal variation in phytoplankton Fe limitation, and that the response of phytoplankton to Mn was subtle and thus readily masked by the responses to Fe. Combined addition of Fe and Mn enhanced carbon uptake of nanoeukaryotes in spring and microeukaryotes in summer while the addition of Mn alone stimulated the growth of picocyanobacteria in autumn. These results suggest the importance of Mn may vary seasonally and its control on phytoplankton growth may be associated with specific taxa.

The Southern Ocean is the largest region in which iron limits the growth of phytoplankton. However, a phytoplankton bloom thousands of square kilometres in area forms each spring-summer in the Indian sector of the Southern Ocean, both above and to the east of the Kerguelen Plateau. The central region of the Kerguelen Plateau hosts the volcanically active islands, Heard and McDonald (HIMI), the former of which is largely covered by glaciers. The sources and processes governing supply of iron from HIMI to the region are relatively unknown. In the austral summer of 2016, the first voyage to focus on biogeochemical cycling in the HIMI region was undertaken (GEOTRACES process study GIpr05). Using iron redox measurements, we show here that each of the adjacent islands are strong sources of dissolved iron(II) (DFe(II)), though controlled by different supply mechanisms. At Heard Island, the greatest DFe(II) concentrations (max 0.57 nmol L) were detected north of the island. An inverse correlation of DFe(II) concentrations with salinity suggests the origin is from a sea-terminating glacier on the island. At McDonald Islands, the greatest DFe(II) concentrations (max 1.01 nmol L) were detected east of the islands which, based on DFe(II) profiles from five targeted stations, appears likely to originate from shallow diffuse hydrothermalism. Elevated DFe(II) around HIMI may increase Fe availability for biota and indicate slower oxidation kinetics in the region, which has implications for transport of Fe away from the islands to the broader northern Kerguelen Plateau where the annual plankton bloom is strongest.