Evidence of Changes in Sedimentation Rate and Sediment Fabric in a Low Oxygen Setting: Santa Monica Basin, CA

The Southern California Bight has grown into the world’s largest urban area during the last century. As a consequence, anthropogenic impacts could disrupt the local marine ecosystem due to municipal and industrial waste pollution and flood control measures. The Santa Monica Basin (SMB), due to its unique setting in low oxygen and high sedimentation environment, has provided an excellent paleorecord of these anthropogenic changes on the Southern California ecosystem. This study examined ten new sediment cores, collected from different parts of SMB in summer 2016, and compared them to previously analyzed cores in order to examine changes in sedimentary dynamics during the last 250 years, with an emphasis on the last 40 years.

Mass accumulation rates (MAR) for the deepest and lowest oxygen-containing parts of the SMB basin (900-910m) established using ²¹⁰Pb have been remarkably consistent during the past century, averaging 17.5 ± 2.1 mg/cm²-yr. At slightly shallower sites (870-900m), accumulation rates showed more variation, but yield the same accumulation rate, averaging 17.5 ± 5.5 mg/cm²-yr. Excess ²¹⁰Pb sedimentation rates were consistent with rates established using bomb-test ¹³⁷Cs profiles. However, ¹⁴C profiles from cores collected in the deepest part of the SMB, where fine laminations are present up to 250 years B.P., indicate that MAR was slower prior to ~ 1900 CE ( rates obtained = 9 and 12 mg/cm²-yr). d¹³C_org profiles show a relatively constant value down core from 0-12 cm depths, suggesting that the change in sediment accumulation rate is not biased by a change in organic carbon sources to the basin. The increase in sedimentation rate towards the recent occurs at about the time previous studies have predicted an increase in siltation and the demise of a shelly shelf benthic fauna on the SMB shelf.

X-radiographs show finely laminated sediments in the deepest part of the basin only, with cm-scale layering of sediments or no layering whatsoever in shallower parts of the SMB basin. The absence of finely laminated sediments in MUC 10 (893 m) and MUC 3 (777 m) suggest that the spread of anoxia has not increased appreciably since cores were last analyzed in the 1980s. Based on core top data collected during the past half century, sedimentary dynamics within SMB has changed minimally during last 40 years. Specifically, mass accumulation rates, laminated sediment fabric, extent of bioturbation, and % C_org have not varied. The only parameter that appeared to have changed in the last 250 years was the MAR with an apparent step-wise increase occurring between ~1850-1900 CE, yet the post-1900 CE constancy of sedimentation through a period of massive urbanization is perplexing.