Abstract
Deep ocean sediment cores collected by IODP Expedition 341 in the Gulf of Alaska record variations in paleoproductivity that provide insight into biogeochemical cycling and ocean circulation on glacial-interglacial timescales. Today, sub-polar North Pacific Ocean surface waters are characterised by high nutrients and low chlorophyll (HNLC) where nitrate, phosphorus, and silica are available in high quantities year-round and primary production is controlled by the availability of micronutrients (iron). Changes in primary productivity related to glacial-interglacial changes provide insight into how sub-polar oceans respond to changing climatic regimes. A continuous Pleistocene sediment record from Site U1417 on the outer Surveyor Fan and ~4200 m water depth, was used to confidently reconstruct these variations in surface productivity during the last 6 high-amplitude glacial-interglacial cycles. A multi-proxy approach, consisting of biogenic silica weight percentages and flux, changes in the isotopic compositions of organic matter (δ13C and δ15N), as well as physical property data, was used to evaluate relative changes in sediment constituents through time. Intervals with elevated δ13C, δ15N, biogenic silica content and flux, as well as decreases in mterr are interpreted to indicate periods of higher productivity combined with reduced influence of glaciogenic sediment delivered by ice rafting, meltwater plumes, and overflow from nearby Surveyor Fan channels. Increases in these paleoproductivity proxies occurred during Marine Isotope Stages 1, 3, 5, 7, 9, and 11 with the highest values during MIS 5 (wt. % BSi – 22.59 %, δ13C - -20.13 ‰, δ15N – 6.12 ‰, mterr – 0.01). When the timing of these results are compared to existing regional records, it appears that productivity increases during deglaciation due to the delivery of bioavailable Fe that is released from the adjacent continental shelves during post glacial sea level rise. At the same time, a decrease in ocean stratification allows for upwelling of nitrate and silica rich deep waters to the surface providing the conditions for an increase in macronutrient fertilisation and primary production. Small increases in productivity that occur during glacial periods may be the result of volcanic ash or an increase in the delivery of glacial dust and ice rafted detritus to the surface ocean. The relationship between deglaciation and productivity is strong in the central Gulf of Alaska, and I conclude that production is the result of increased nutrient abundance and transport, as well as upwelling that occurs in response to retreating continental ice and increasing sea surface temperature.