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UBC Theses and Dissertations

The late Quaternary history of primary productivity in the subarctic east Pacific McDonald, Darcy


Palaeoceanographic work in the subarctic northeast Pacific has previously been limited by the common presence of turbidites and by the relatively shallow CCD. ODP Site 887 is on the Patton Murray Rise, an elevated plateau which is largely isolated from these effects. This area is centered under the Alaska Gyre, a region characterized by the domal upwelling of nutrient-rich waters. Sections from Holes 887A and 887C were spliced into the 887B profile to obtain a continuous composite record. Although calcite is not continuously distributed throughout the drilled section, the benthic foram 6¹⁸O profile from this record closely reflects the global (SPECMAP) signal. Hence, based on 6¹⁸O data and on radiocarbon dates from site survey cores,* a detailed chronology spanning the past 750 ky was developed. The controls on productivity in this region remain unclear since nitrate is perennially present in excess and does not limit primary production. However, past increases in productivity and rapid settling of biogenic matter are suggested by the episodic accumulation of diatomaceous oozes, and by high-resolution Si/Al, Ba/Al, C¹³org, and CaC0₃ profiles. Moreover, episodic shoaling of the redox boundary produced by increased organic matter flux to the seafloor, as during the deposition of the diatomaceous oozes, is suggested by trace metal and Mn enrichments. Significant 6¹³Corg maxima in the major diatomaceous bands suggest that productivity events have occurred which were sufficient to draw down mixed-layer PCO₂. Thus, in contrast to the near-neutral ocean-atmosphere PCO₂ gradient which exists today, an episodic sink for atmospheric CO₂ may have existed in the Gulf of Alaska in the past. Surface water temperature and salinity effects are evident in negative regional surface water 6¹⁸O variations. The episodes of enhanced productivity at Site 887 occur synchronously with such 6¹⁸Olocal excursions, suggesting a direct link to low salinity and/or warming events in the Gulf of Alaska which occur on sub- Milankovitch timescales. This suggests that significant climate variability in the Gulf of Alaska has been superposed on glacial-interglacial cycles through the Brunhes Chron. Temporal variations in the availability of iron may also have played a key role in constraining export production through time. Episodic input of iron, which may have stimulated productivity, may have occurred from either Asian sources via the atmosphere or more local sources via meltwater input.

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