Pre to Post-Bomb Seawater 14C History in the Gulf of Alaska Inferred From a Deep Sea Coral: Isididae sp.

Deep-sea corals are an important archive of intermediate and deep-water variability, and provide the means to explore decadal to century-scale ocean dynamics in regions and time periods heretofore unexplored. We present a reconstruction of pre to post-bomb surface and interior water Δ14C based on analysis of deep-sea Isididae (bamboo) corals collected live at ~700 meters in June 2002 at Warwick Seamount, Gulf of Alaska. Concurrent isotope analyses of polyp/tissue and outermost portion of the hard horny proteinaceous gorgonin nodes compared with in situ dissolved inorganic carbon indicates that the gorgonin portion is derived exclusively from recently fixed/exported particulate organic carbon and thus a record of the surface water 14C/12C history. This is in contrast to the carbonate internode portion which is primarily derived from in situ dissolved inorganic carbon, and thus a record of the in situ 14C/12C. Radiocarbon analysis of gorgonin nodal sections captures the surface water D14C evolution. Pre-bomb values are -105‰ reaching a maximum of 100‰ before decreasing to collection values of 20‰. We anticipate that the post-bomb maximum will be in the early 1970s consistent with other mid to high latitude records and that the pre/post bomb transition initiates near 1956. If we utilize the gorgonin pre/post bomb transition as a tie-point and assume a linear growth rate the Isididae used in this study are 75- 125 years old. Carbonate Δ14C shows a 25‰ increase from -215 to -190‰ reflecting the penetration of bomb-14C in the sub-polar North Pacific. To place the carbonate time-series on a fixed timescale we determined the minor element chemistry and tested the inter-species reproducibility. The distribution of Sr is quite homogenous whereas Mg is not with higher Mg concentrations associated with centers of calcification. Age estimates using what appear to be annual Sr/Ca cycles, which we hypothesize are related to biomineralization cycles associated with a large seasonal cycle in POC, are consistent with the gorgonin 14C derived ages. Using the Sr/Ca variations as an independent age model allow the carbonate D14C results to be placed on a firmer time-scale and constrain the absolute age-corrected D14C history of interior water masses. We estimate pre-bomb surface (age corrected) Δ14C at Warwick to be ~-97‰. This value is somewhat more depleted than both silicate and PALK derived estimates of surface water pre-bomb Δ14C. The post-bomb maximum was ~+110‰. In addition we have reconstructed the Δ14C history at 634 and 720m at Warwick seamount using the carbonate internodes. The results document a small ~10‰ pre-bomb gradient between 634 and 720m. The largest feature in the record is a 20‰ increase in Δ14C that we interpret as the penetration of bomb-14C out of surface waters and into the interior of the northeast Pacific. At 634m this maximum "anomaly" occurred in the early 1980s and a decade later at 720m. At 720m we document a 10‰ decrease in Δ14C in the early 1970s. We interpret this feature as entrainment of low-14C water from deeper depths, either locally or upstream of Warwick Seamount.