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

Abstract. The Southern California Bight is adjacent to one of the world's largest urban areas, Los Angeles. As a consequence, anthropogenic impacts could disrupt local marine ecosystems due to municipal and industrial waste, pollution, and flood control measures. Superimposed on the growth of an urban metropolis, the impact of climate change has been felt most strongly over the past 50 years in terms of rising pCO2 and warming. Santa Monica Basin (SMB), due to its unique setting in low oxygen and high sedimentation environment, has provided an excellent sedimentary paleorecord of these anthropogenic changes. This study examined ten sediment cores, collected from different parts of the SMB between spring and summer 2016, and compared them to existing cores in order to document 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 (900–910 m) established using 210Pb have been remarkably consistent during the past century, averaging 17.5 ± 2.1 mg/cm2-yr. At slightly shallower sites (870–900 m), accumulation rates showed more variation, butyield the same accumulation rate, 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). δ13Corg profiles show a relatively constant value down core suggesting that the change in sediment accumulation rate is not accompanied by a change in organic carbon sources to the basin. The increase in sedimentation rate towards the recent occurs at about the time previous studiespredicted 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 rate at which anoxia is spreading, 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 % Corg have not changed. 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 surprising.