UBC Theses and Dissertations
Palaeoceanography of the Northeastern Pacific Ocean off Vancouver Island, Canada McKay, Jennifer Lynn
Marine sediment cores from the continental margin off Western Canada (48° to 50°N, 125 to 128°W) yield evidence of dramatic changes in oceanographic conditions over the last 16 kyr. During the late Glacial the accumulation of marine organic matter was greatly reduced. This reflects low primary and export production because glacial-mode atmospheric circulation did not drive coastal upwelling. At the start of the Boiling warm period (-14.3 kyr B.P.), and coincident with the retreat of glaciers from the continental shelf, there was a substantial increase in the burial of organic matter. However, much of this material was "old" terrestrial organic detritus derived in part from the erosion of shelf sediments. It was not until the Allerod (-13.5 to 12.6 kyr B.P.) that the accumulation of marine organic matter increased substantially. Other paleoproductivity proxies (i.e., % biogenic Ba, % opal and alkenone abundances) also indicate very high marine productivity at this time. Since primary production in the region is controlled by the upwelling of nutrient-rich subsurface waters, this suggests upwelling was enhanced during the Allerod. A decrease in benthic-planktonic age differences (i.e., older planktonic foram ages) supports this interpretation. There was a brief return to glacial conditions (i.e., lower primary and export production) during the Younger Dryas, followed by a slight rise in organic carbon burial between -11 and 10 kyr B.P.. In general, the accumulation of organic carbon was low throughout the Holocene despite high primary productivity. Low sedimentation rates and the resulting long oxidant exposure times as well as extensive biological recycling appear to be the primary causes of low organic carbon burial, although lower productivity relative to the Allerod cannot be ruled out. Changes in the vertical settling flux of organic carbon to the sediment had a direct impact on sedimentary redox conditions at Site JT96-09 (920 m water depth). At present, near-surface sediments become suboxic within millimetres of the sediment-water interface and Re enrichment is observed below the depth of bioturbative mixing. However, anoxic conditions do not develop despite relatively low bottom water oxygen concentrations (0.3 ml/1) and relatively high organic carbon flux to the sea floor. The situation was much different during the deglacia During the Allerod, when the organic flux to the sediment peaked, Mo enrichment is observed (>2 ug/g) implying that near-surface sediments were anoxic at this time. The benthic foraminifera assemblage (i.e., the dominance of Bolivina spp.) likewise suggests that sediments were more oxygen-depleted. These results imply that the bottom waters at Station JT96-09 were more oxygen-depleted during the Allerod and since JT96-09 is located within the oxygen minimum zone (OMZ), it follows that the OMZ was more intense. The increase in marine organic carbon accumulation and the lack of evidence of decreased ventilation suggests that OMZ intensification was a response to increased primary production and carbon export. Deglacial intensification of the OMZ is documented along the entire length of the California Current System, but it appears to have been delayed by ~1500 years off Vancouver Island. It is probable that the presence of the Laurentide and Cordilleran ice sheets continued to influence atmospheric circulation, and in turn oceanic circulation, in the region until at least the Allerod. It is possible to use redox-sensitive trace metals as palaeo-proxies in the deglacial sediments of Core JT96-09 because sedimentation rates were quite high (>100 crn/kyr). In contrast, in the slowly deposited Holocene sediments (5 cm/kyr) the trace metal record is corrupted due to the precipitation of trace metals decimetres below the sediment-water interface. This enrichment obscures the original palaeo-signal, which formed shortly after sediment deposition. Rapid fluctuations in the sedimentation rate can also complicate trace metal records. In the deglacial deposits of Core JT96-02 the formation of Mo and Re concentration spikes is the direct consequence of episodic deposition of turbidites that restricted oxygen influx into the sediment and thus allowed anoxic conditions to develop. Intriguing results were obtained from the measurement of sedimentary Ag concentration in near-surface sediments. Unlike the other redox-sensitive trace metals, for example Cd which has a similar geochemical behaviour, the primary control on Ag concentration is not sedimentary redox conditions. Rather, it seems that Ag accumulates in, and is transferred to the sediment by, settling organic particles in much the same way that Ba is. Hence, measuring the Ag concentration of sediments may provide another means of assessing palaeoproductivity.
Item Citations and Data