The objective of this study was to compare two techniques for estimating benthic fluxes of nutrients (nitrate, phosphate, and silicic acid) and Ge/Si flux ratios. In situ flux chambers were deployed, and cores were collected and incubated at 9 sites along the California margin in July 2001. Both techniques were successful at 8 sites, at depths from 100 to 3300 m. Flux chambers were deployed for 1 to 2 d, and cores were incubated for slightly longer on board the ship in a cold room. In some cases, core incubation flux temperature varied by up to 5°C from in situ temperature, and core incubation results were adjusted for this factor based on the effects of temperature on diffusivities and the adsorption of silicic acid. Sites studied had a range in nutrient fluxes of more than an order of magnitude, based on in situ chambers. The temperature‐adjusted core incubation fluxes showed a similar, but slightly smaller range. Both methods had similar precision based on replicates, with uncertainties for high flux stations that were 5% to 20% of the mean. Only phosphate showed significant (95% confidence level) spatial variability in replicate cores; the larger in situ flux chambers had less spatial variability. The two techniques did show some systematic differences that are attributed to several artifacts created by core recovery. Silicic acid fluxes from cores were significantly lower than in situ fluxes at 2 sites; overall averages were about 80% of those for in situ chambers. The differences are attributed to reduced macrofaunal irrigation in incubated cores. Nitrate uptake in core incubations at 5 of 8 stations was significantly lower than in situ uptake; shipboard rates for all sites averaged about 66% of in situ chamber rates. This difference is attributed primarily to decreased denitrification rates in recovered cores in response to altered temperature and pressure. Phosphate fluxes from cores were significantly lower at only one site; overall, results for the two techniques were indistinguishable. Only one site had a significantly different Ge/Si flux ratio.
Chemical analyses of the clay-sized fractions of 564 continuous sediment samples (200-yr resolution) from composite core OL90/92 allow quantification of an abundance of glacial rock flour. Rock flour produced during glacier advances is represented by clay-sized plagioclase, K-feldspar, and biotite in homogeneous internal composition. The abundance of rock flour is deemed proportional to the intensity of glacies advances. Age control for the composite section is provided by combining previously published radiocarbon dates on organics, U/Th dates on ostracode shells, and U/Th dates on saline minerals from nearby Searles Lake correlated to OL92 by pollen. The rock flour record displays three levels of variability: (1) a dominant one of about 20,000 yr related to summer insolation and precipitation; (2) an intermediate one of 3000–5000 yr, perhaps related to North Atlantic Heinrich events; and (3) a minor one of 1000–2000 yr, perhaps related to North Atlantic thermohaline-driven air-temperature variation.
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