Historical Trends of Metals in the Sediments of San Francisco Bay, California:
Core data from San Pablo Bay, Grizzly Bay, Richardson Bay, and Central Bay
by Michelle I. Hornberger, Samuel N. Luoma, Alexander van Geen, Christopher Fuller, and Roberto Anima, USGS
based on article published in Marine Chemistry, 1999. V. 64, pp 39-55.

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Methods: Dating

Analyses of ¹³⁷Cs, ²¹⁰Pb, ^239,240Pu, and ²³⁴Th were conducted on cores RB92-3 and SP90-8 to derive chronologies (Fuller et al, 1998). Additional analyses of ¹⁰Be constrained early human activities on each core and ¹⁴C was used to identify the oldest sediments in RB92-3 (van Geen et al., 1998). Sediments in RB92-3 appeared to be continuously deposited since well before significant anthropogenic activity began in the watershed (van Geen et al., 1998). The linear sedimentation rate at the surface of RB92-3 was 0.89 cm/yr and the core was vertically mixed to 33 cm depth (Fuller et al., 1998). Dates of sediment deposition were determined by numerical simulation of ²¹⁰Pb profiles. The dates on individual horizons are the minimum age of sediments at that depth. The deposition rate in SP90-8 averaged 4.1 cm/yr based on ¹³⁷Cs and ^239,240Pu activity maxima and ²¹⁰Pb profiles. Profiles of ¹³⁷Cs were also determined in three additional cores, in order to estimate the depth of sediment deposition since 1952 ± 2 (method described by Fuller et al., 1998).

Jaffe et al. (1998) reconstructed depositional processes at SP90-8 by comparing five detailed bathymetry surveys conducted since 1850. A discontinuity in chemical concentrations observed at ~120 cm depth in this core appears to coincide with a depositional hiatus that extended from 1880 to 1950. Both depositional history and Sr/Nd isotopic signatures suggest sediments between 150-250 cm originated from hydraulic mining activities and were deposited between 1850 and 1880 (Jaffe et al., 1998; Bouse et al., 1996). Thus, sediments deposited before 1850 lie directly beneath sediments deposited in the 1950's.

Inventories of ‘excess’ metal (mass per area deposited in excess of baseline) were determined in GB90-6, SP90-8, and RB92-3 by integrating contaminant metal inputs downcore. The µg/cm² of metal in interval a (M_a) was determined by

M_a = (C_a-B) * p_z * _z        (1)

where C_a is the metal concentration in µg/g in interval a, B is the baseline metal concentration that occurred before anthropogenic activities began in the watershed (see later discussion), p_z is the bulk density of the sediment, and z is interval thickness in cm. Fuller et al. (1998) reported pz for each horizon in RB92-3; an average bulk density of 1.1 g/cm2 was used for SP90-8. The integrated inventory for sediments (_) was determined by

_ = _(C_a-z - B),       (2)

for all intervals. Intervals not sampled were assigned values by linear interpolation of the concentration (mass of metal/g) of adjacent intervals. Long-term mean excess metal flux, _, was compared among cores with different sedimentation rates. To do so, _ was divided by the number of years of excess metal input (e.g., normalized to the period of human disturbance) using age estimates from Fuller et al. (1998)