Seasonal time series profiles provide insight into iron sources to Gulf of Alaska surface waters

The Gulf of Alaska is one of the largest iron-limited regions of the ocean, yet our quantitative understanding of the sources of iron (Fe) remains poor. One means of evaluating Fe inputs to a region is from time-series data, which can reveal insight not easily gleaned from a single research cruise. Re-examination of previously published dissolved iron profiles from Ocean Station Papa in the central Gulf of Alaska reveals 33-70% increases in the dissolved iron inventories occurring between September and February of two successive years. We can virtually rule out many possible iron sources at this time of year. However, the timing of the inventory increases coincides with known inputs from dust originating from glacierized river valleys of southern Alaska, suggesting this dust as a plausible source of the added Fe. This suggestion requires validation, but if true, this would imply this Alaskan dust is transported ≥1100 km from the coast, more than twice as far as has been visually documented from satellite observations. This possible Alaskan glacial dust Fe source has not received much attention in part because state-of-the-art global dust models fail to reproduce this flux. This is because the model spatial resolution is too coarse by an order of magnitude to resolve the high winds that generate the dust from narrow mountain river valleys. Another reason this process has not received much attention is that iron inputs in the autumn or early winter may not generate a phytoplankton bloom in direct response, because productivity may be light-limited. Questions remain regarding the biological impact of this iron, which the data suggest is transported to intermediate depths by processes that could plausibly include abiological scavenging and particle sinking. The data and their possible implications reveal the value of time-series profiles of Fe (not just surface waters) as a tool to understand both the drivers and consequences of iron inputs to the ocean.