Late Holocene sediment accumulation on the northern California shelf: Oceanic, fluvial, and anthropogenic influences

Abstract

The late Holocene sedimentary record of the northern California continental shelf archives the combined infl uences of fl uvial and oceanic mechanisms of land to ocean sediment fl ux, and provides perspective on the cumulative impacts of major fl oods and land-use change in the drainage basin during historical time. Piston cores collected on the shelf (50−150 m water depths) off the Eel and Klamath Rivers were analyzed to identify lithologies indicative of depositional processes over the past 5 k.y., and dated using 137 Cs and 14 C methods to resolve spatial patterns in sediment accumulation rates. Sediment accumulation rates averaged over the past several millennia display along-and across-shelf gradients that refl ect interactions between the shelf fl ow fi eld and antecedent tectonically produced topography. The highest rates (3−6 mm/yr; 0.4−0.8 g/cm 2 /yr) are associated with structural lows on the middle shelf, whereas the lowest rates (0.2−0.6 mm/yr; 0.03−0.05 g/cm 2 /yr) occur at structural highs and at the shelf edge. This accumulation pattern is spatially coincident with postglacial sediment thicknesses on the shelf, and with depocenters that form on shorter time scales in response to oceanic fl ood events. The sediment column exhibits an upward-fi ning sequence consistent with increased bypass of river silt and clay as delta plains progressively fi lled during the Holocene transgression. Sedimentary event beds indicative of major river fl oods, including two probable hyperpycnal discharges of the Eel River within the past 1 k.y., are preserved. Stratigraphic evidence of accelerated sediment delivery to the shelf during historical time, in the form of increased burial of fl uvial mud relative to fi ne-grained sand, is superimposed on this natural record. This change is concordant with the regional history of forest clearing and related land-use practices in the watershed after the early 1800s, and appears to record the combined impacts of timber harvesting and several extreme fl oods after ca. A.D. 1950.

A new look at old carbon in active margin sediments

“…1). Average sedimentation rates are nonetheless approximated by interpolation of ln( 210 Pb xs ) versus depth to be 1.1 and 0.6 cm yr -1 over the length of GGC5 and MC36, respectively, in broad agreement with other estimates for the mid-shelf depocenter (Leithold et al, 2005;Sommer fi eld and Nittrouer, 1999;Sommerfi eld and Wheatcroft, 2007). As gravity coring usually displaces several centimeters of surfi cial sediments, the depth scale for GGC5 was linked to the seafl oor via comparison of its 210 Pb xs and 137 Cs activities to those of MC36.…”

Environmental signal propagation in sedimentary systems across timescales

“…Characteristics of deep-time sediment production and 1176 transfer areas can be interpreted by employing provenance tools, detrital mineral analysis, 1177 or application of empirical relationships based on modern systems, and tested with 1178 conceptual, analytical, and numerical models. Sommerfield and Wheatcroft, 2007). 1199…”

“…The character of that signal and to what extent a signal is 1144 preserved in sedimentary records are dependent on the magnitude and frequency of the 1145 initial forcing (Fig. 8), on their initial recording or destruction (Wheatcroft et al, 2007), The investigation of signal propagation requires a systems approach, which is 1163 provided by the sediment-routing system, or source-to-sink, framework (Allen, 2008a) 1164 (Fig. 1).…”

The Continental Shelf and Continental Slope

“…There is some input of finer grained sediment from plumes and nepheloid layers; although it used to be considered that these volumes were comparatively small, recent detailed sediment budgets have suggested that they may be substantial. Thus, for example, Sommerfield and Wheatcroft (2007) found that 20% of terrigenous sediment delivered to the ocean from the Eel River in California accumulates on the outer shelf, although note that this is a narrow shelf environment (c. 12-17 km). A small number of outer-shelf environments are affected by the effects of ocean currents; examples include the outer Saharan shelf, the Middle Atlantic Bight and the southeastern African shelf.…”