THE ROLE OF THE ALASKA COASTAL CURRENT IN HOLOCENE PRIMARY PRODUCTIVITY IN SOUTHEAST ALASKA FJORDS

The biogenic sediments preserved in the temperate ice-free fjords of Southeast Alaska offer an opportunity to better document the distribution of Holocene phytoplankton productivity in the Gulf of Alaska (GoAK), as well as study the interplay of micronutrient-rich waters from the Alaska Coastal Current (ACC) and subsurface High Nutrient Low Chlorophyll (HNLC) waters that are advected from the open GoAK into coastal Southeast Alaska. A Holocene synthesis of biogenic silica (opal) and organic matter δ¹³C data from sediment cores recovered in four fjords shows evidence of two significant diatom productivity intensifications at ~3.5 and ~1.0 cal ka BP. The timing of these productive intervals corresponds closely with declines in opal seen along the edge of the GoAK continental shelf. This difference between the fjords and offshore GoAK may be explained by a coastal micronutrient capture mechanism (“the Iron Curtain”) where enhanced coastal productivity reduces the availability of continental weathering-derived micronutrients that are exported by the ACC to the HNLC waters of the open GoAK shelf which limit offshore primary productivity.

These phytoplankton patterns are compared against geochemical proxies for both nearshore and offshore ACC strength. X-ray fluorometry (XRF)-based proxies of detrital run-off from the coastal fjord records indicate the highest run-off rates during the early Holocene, which decline to near-modern levels between 6 and 7 cal ka BP. An offshore ACC record reveals strong ACC flow during the early Holocene (>8 cal ka BP) and oscillatory behavior during the middle and late Holocene (~6 to <1 cal ka BP). Taken together, the offshore and nearshore ACC records suggest: (1) a shared mechanism affecting the ACC during the early Holocene, possibly related to the decay of the Cordilleran Ice Sheet and coastal release of stored sediments; and (2) the divergence in nearshore and offshore ACC patterns during the middle and late Holocene imply the nearshore ACC is less dynamic relative to the offshore sector of the ACC. These findings imply coastal fjord productivity in Southeast Alaska likely responds to non-ACC influences, such as the subsurface delivery of macronutrient-rich HNLC waters from the open GoAK, which is in turn influenced by regional atmosphere-ocean interactions.