Biological uptake, water mass mixing and scavenging prevent transport of
manganese-rich waters from the Antarctic shelf
Abstract
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.