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

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 15 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 (900-910m) established 20 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, 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). d13Corg profiles show a relatively constant value 25 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 studies 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 30 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 https://doi.org/10.5194/bg-2019-420 Preprint. Discussion started: 6 November 2019 c © Author(s) 2019. CC BY 4.0 License.


Introduction
The use of laminated sediments as a record of environmental change has many historical precedents (Koivisto and Saarnisto, 40 1978;Gorsline, 1992;Algeo et al., 1994). The deepest portion of Santa Monica Basin (SMB, Fig. 1) has been accumulating finely laminated sediments for the past approximately 400 years (Christensen et al., 1993). The presence of fine lamination is evidence that macrofaunal activity on or in the sediment has been minimal to absent. Savrda et al. (1984) documented the transition from laminated to bioturbated sediments as corresponding to a change in oxygen concentration in the bottom water, which is the chief control of benthic macrofauna presence (Levin, 2003). Yet two things are necessary to produce laminated 45 sediments. First is the absence of disturbance or mixing, and the other is a pulsed delivery of sediment that produces a distinction in composition or sediment fabric (Kemp, 1996).
Sediment trap studies at a long-term study site (SPOT, Fig. 1) in adjacent San Pedro Basin demonstrated a seasonal pattern of sedimentation with highest rates in late winter and spring (Collins et al., 2011). Similarly, Haskell et al. (2015) documented 50 seasonality in upwelling velocity and biogenic particle export from the upper ocean at SPOT. In contrast to the annual forcing of sedimentation in local waters, sediments in the SMB show primarily non-annual laminations with a frequency of 3-7 year.
This lamination cycle may be consistent with the frequency of heavy rainfall in Southern California during El Nino years (Quinn et al., 1978;Christensen et al., 1993) 55 The present study considers changes in SMB sedimentation over the past 150 years, a time period when changes in ocean biogeochemistry have been observed both globally and regionally. For example, the large-scale changes in the size and intensity of global oxygen minimum zones (Stramma et al., 2010;Brietburg et al., 2018) have also been documented for Southern California waters (Bograd et al., 2008). Oxygen concentrations in near-surface waters of the Southern Californian shelf show a 20-to 50-year decline beginning in the early 1960s, which was attributed to increased stratification and/or 60 increased productivity caused by enhanced nutrient supply (Booth et al., 2014). In concert with changes in upper ocean oxygen content, research by Huh et al. (1989) and Christensen et al. (1993) have documented expansion of the area of laminated sediments in SMB over the past 400 years. Their work with X-radiography showed that homogenized sediment was covered by laminated sediment, marking a transition from bioturbation to lamination preservation.

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Age dating of this transition, as deduced by applying a 210 Pb-derived sedimentation rate, revealed concentric zones covering the entire basin floor, which accumulated laminated sediments from 400 (basin center) to 50 (shallower depths) years before present (ybp). This expansion in lamination is taken as evidence of expanding oxygen deficiency and is particularly interesting given the global and local changes mentioned above. Over the past 400 years of laminae accumulation in SMB, the southern California region has grown into one the world's largest urban areas. Particularly notable was a change in ecosystem structure 70 of benthic shelf fauna during the mid-to late 1800's, which was attributed to an onset of higher coastal sediment delivery caused by grazing cattle (Tomašových and Kidwell, 2017). This new land use was proposed to increase the frequency and amount of sediment entering the coastal zone. Another notable anthropogenic impact is the introduction of sewage waste into the coastal system starting in the early 1900's (Alexander and Venherm, 2003). Advanced treatment of this sewage did not start until the 1970's. Furthermore, channelization of the LA River and construction of sediment-trapping flood basins up-river 75 have occurred over the past century (See supplement section for LA land usage timeline). Thus, there is ample evidence of environmental change in and around the SMB over the past 150 years.
Starting with a study by Bruland et al. (1974), investigators have been using 210 Pb profiles of sediments as a means of documenting sediment accumulation and sediment mixing in the SMB. A compilation of core analyses was published by Huh 80 et al. (1989), and further work by Alexander and Lee (2009) provided a record of sedimentation in the SMB from the 1970's through the 1990's. Our work here (conducted in 2016) aimed at augmenting this record of coastal sedimentation, quantified by analyses of 210 Pb and 14 C profiles. We sampled intact surface sediments (top ~30 cm) and also conducted analyses of (1) sediment fabric by x-radiography, (2) sediment macrofaunal composition, and (3) Corg content to study changes in sedimentation in the SMB over the past 150 years. Our study provides new information about sedimentation and the potential 85 expansion or contraction of laminated sediments over the past 150 years with a focus on the past 40 years.

Study Area 90
The San Pedro-Santa Monica Basins are 'bathtub'-shaped basins, oriented north-west to south-east adjacent to the Los Angeles coastline. They are both approximately 900 m deep, separated by a sill. Water entering San Pedro Basin (SPB) from the southeast crosses the sill at ~740 m and then passes into the SMB (Hickey, 1991). Bottom water circulation below the sill depth is sluggish, < 1.0 cm/s and generally moves in a counter-clockwise direction. To the north-east of SMB is a slope and the broad 95 Santa Monica shelf, which is incised by Redondo Canyon, in the south-east portion of the basin, and Santa Monica Canyon, which empties in the middle of SMB; Malibu and Pt. Dume Canyons drain into the northeast portion of the basin.
Sedimentation is characterized as hemi-pelagic, interrupted by sandy turbidites that primarily originate from the northeast canyons and spread onto the basin floor (Gorsline, 1992).

100
The upper ocean waters (above 300 m) are a mixture of at least two distinctive water masses (Fig. 2) whereas the waters below sill depth have a T-S signature suggestive of mixing with a water mass that originates somewhere in the NW Pacific (Lynn 4 and Simpson, 1987). All waters below 400 m are low in oxygen (<20 µM) although the deepest water sometimes has slightly higher concentrations compared to those immediately above (Fig. 2). This phenomenon is rare and identifies a basin 'flushing' event (Berelson, 1991). Generally, water enters SMB, and the sluggish circulation and slow rates of replenishment (deep water 105 residence times on the order of 1-3 years; Hammond et al., 1990) tend to deplete oxygen further. Hence oxygen concentrations range between 1 -9 µM (Berelson, 1985). Complete bottom water depletion of oxygen and/or the presence of sulfide in bottom waters has never been reported. The sediments of SMB have 15-20 wt. % CaCO3, 2-6 wt. % Corg and 2-8 wt. % SiO2 (Cheng et al., 2008). Ten sediment cores were collected in April and July 2016 from eight stations (MUC 3, MUC 5-11) between 319 and 907 m water depth, using a miniature multicorer (MUC, K.U.M. Kiel) equipped with four polycarbonate core liners (length: 60 cm, inner diameter: 9.5 cm). 120 After cores were retrieved, one core was sectioned on shipboard in one-centimeter intervals through the upper 10 cm and twocentimeter intervals below 10 cm. Aliquots were sealed in porosity vials, and the remaining mud was placed in plastic bags.
A second core from the same multicore deployment was preserved intact for x-radiography.

Porosity and Integrated Mass
Wet mud from the sectioned core was placed in pre-weighed porosity vials (15 mL snap-cap glass vials), re-weighed and dried at 50°C for 48-96 hours. Vials were subsequently re-weighed to determine water loss. The dry weight was corrected for salt content, assuming a salinity of 35. Porosity was determined assuming a grain density of 2.5 g/cm 3 . Integrated mass to the mid-130 point of each sample interval was calculated from the porosity profile and this density, summing to numerically integrate eq. 1: 135 where dx is the interval thickness, r is solid phase density, I is integrated mass, and ∅ is porosity.

Macrofauna
Sediment from one of the cores collected from each site was used for faunal surveys. The first 5 cm of each core was sectioned 140 into 1 cm intervals for the purposes of capturing faunal variability near the sediment water interface. The remaining length of the core was sectioned into 2 cm intervals. The sediments from each interval were then washed with DI water through a 2mm sieve, and the residue collected. Macrofauna and meiofauna in each section were identified with the aid of optical light microscopy, and were preserved in an ethanol-glycol mixture (80% ethanol).

Organic Carbon Content
Dried porosity samples were ground by mortar and pestle and this homogenized sediment was used for Corg, 210 Pb and 137 Cs analyses. A portion of the ground sediment was weighed (10-150 mg) and was placed into a 10 mL exetainer tube and acidified with 10% phosphoric acid. The evolved gas was analyzed for CO2 using a Picarro CRDS, following procedures developed at 150 USC (Subhas et al., 2015(Subhas et al., , 2017. This provided a measure of acid-reactive C, assumed to equal C bound as CaCO3. Another split of powder was weighed into tin capsules and combusted at 800° C on a Costech CN analyzer to measure total carbon, with the CO2 and d 13 C concentration also determined via the Picarro. USGS standards were used to calibrate wt. % Total C in samples. The difference between total C and CaCO3 carbon was taken as the % Corg. Replicates indicate analytical uncertainties in this measurement of ±0.2 wt. % Corg on samples that have 2-6 wt. % Corg. 155

Photographs and X-radiographs
Replicate cores from each multicorer sampling were photographed at University of California Los Angeles (UCLA). Cores returned to the University of Southern California (USC) and were stored for 2-4 months to air-dry, which allowed the sediment 160 to lose water and consolidate. A router was used to remove a section of plastic core liner on opposite sides of the core tube.
The core was split into two halves with smooth cut faces from top to bottom using a wire. One split core was transferred to a plastic tray with approximately a 2cm lip along the long edges. The wire was run along the top of the lip, yielding a uniform 2 cm thick slab of sample. Each slab was placed on a large sheet of Kodak film and x-rayed for 90-180 sec at 8 milliamps and 96 volts. Negatives were developed in a dark room. Approximately 0.5-1.0 g of dried, homogenized sediment was placed in 5 mL polypropylene test tubes for analysis by gamma spectroscopy. Excess 210 Pb and 137 Cs activities in sediments were measured using high purity intrinsic germanium well-type detectors (HPGe ORTEC, 120 cm 3 active volume). Detector efficiencies were determined by counting the activities of known standards in the same geometry as the samples. Standards used included IAEA-385 marine sediments, EPA Diluted Pitchblende SRM-DP2, and NIST 210 Pb liquid solution. Samples were counted for 2-4 days, and the spectra (keV) were 175 analyzed for the following radioisotopes: 210 Pb (46), 214 Pb (295)

185
Radiocarbon values were measured using the accelerator mass spectrometry (AMS) at the University of California Irvine (UCI) Keck Carbon Cycle Accelerator Mass Spectrometry (KCCAMS) laboratory. Samples were subjected to HCl vapor for four hours to acidify calcium carbonate, dried on a vacuum line, combusted, graphitized and then counted on the AMS. Sample preparation backgrounds were subtracted, based on measurements of acidified glycine, ANU and Lycine. Radiocarbon results have been corrected for isotopic fractionation according to the conventions of Stuiver and Polach (1977), with 13 Corg measured 190 using a Costech ECS 4010 Analyzer -Delta V Plus IRMS at the University of California Riverside (UCR). The isotopic ratio is given in delta notation relative to Vienna Pee Dee Belemnite (VPDB) for 13 C values. Glycene, peach, acetate and house soil were used as reference material, standard error (Ơ ) was <0.10‰.

Results
Sediment porosity declined with depth, with generally higher values in cores collected at deeper stations (Fig. 3). At all sites, there was typically a porosity difference of ~0.2 between the sediment-water interface (SWI) and 30 cm depth horizon. 200 However, several cores showed notable interruptions in the monotonic decline in porosity with depth. Core MUC 9 and MUC 10 had intervals with lower porosities compared to the overall depth trend. Low porosity anomalies were observed below 25 cm in MUC 9 and at 13-15 cm, ~22, and ~28 cm in MUC 10.

7
Only three cores (of those collected at depths > 320 m) had macrofauna obtained from sediment sectioning and sieving (Table  205 1). Notable was the abundance of sponge spicule clusters found throughout much of core MUC 8. An intact annelid worm was found on the surface of MUC 11 (745 m), which had oxygen concentrations < 8 µM near the seafloor.
Weight percent Corg content of the upper cm of the cores collected in 2016 showed a distinct trend of increasing %Corg with water depth (Fig. 4). Basin sediments (MUC's 9 and 10) had 5-6 wt. % Corg whereas slope sediments ranged from 2-5 wt. %. 210 Cores collected in the 1970's and 1980's show the same trend for core top %Corg vs. water depth as the MUC cores (Gorsline, 1992;Fig. 4).
Photographs of MUC cores showed light reddish-brown colored sediment near the surface of each core and a progression in MUC 9 and 10 toward darker colored sediment with depth (Fig. 5). Only MUC 9 (907 m) had laminations visible by eye. The 215 sediment in the upper 10 cm from other cores (MUCs 10, 3, and 11) appeared homogeneous. MUC 11 showed a living polychaete worm present at the sediment-water interface.
X-radiographs of MUC 9 and MUC 10 revealed distinct laminations (Fig. 6). MUC 9 showed clear sediment laminations down to approx. 15 cm. However, MUC 10, collected from a site only 14 m shallower did not show fine lamination, but broader 220 banding was apparent down to 12 cm. Both cores showed zones of higher density material (light-colored in x-radiograph negative). A distinct higher density zone is seen in MUC 9 below 25 cm. Three zones of dense material were detected in the MUC 10 x-radiograph; the first was between 12-17 cm, the second at ~22 cm, and the third below 28 cm. These zones of higher density material correspond with the zones of anomalously low porosity (Fig. 3).

Excess 210 Pb and 137 Cs
Values of excess 210 Pb in surface sediments varied from 25 dpm/g at the shallow water sites to 100 dpm/g in deeper waters near the mid-basin (Fig. 7). Many of the cores from the shallower sites (<800 m) showed a constant activity of excess 210 Pb 230 in the top 1-3cm, below which activity decreased exponentially. MUC 8 deviated from this trend and showed an increase in excess 210 Pb at 9 cm. MUC 9 and MUC 10, which are the two cores in the central basin collected from water depths greater than 850 meters, showed high values of excess 210 Pb near the surface and an exponential decrease below the sediment-water interface. Excess 210 Pb in these two cores was restricted to the top 8 cm, whereas excess 210 Pb penetrated deeper into the sediment of cores from the basin slope (MUC's 5, 6, 7, 8, and 3). 235 8 137 Cs profiles of MUC 9 and MUC 10 showed peaks between 4.5 and 2.5 cm depth, respectively (Fig. 8), whereas 137 Cs profiles of cores taken along the slope showed very low values with large uncertainties.

Radiocarbon and d 13 Corg 240
The organic carbon from selected intervals from MUC 9 and MUC 10 was measured for radiocarbon content and d 13 Corg to depths of 25 centimeters (Fig. 9, Fig. 10). ∆ 14 C (BP)* and d 13 Corg values were plotted vs. integrated mass to provide a normalization for thedowncore porosity changes that occur downcore. ∆ 14 C (BP)* indicates a conventional radiocarbon age that was determined using the method of Stuiver and Polach (1977). A reservoir age adjustment was not applied to the ∆ 14 C 245 (BP)* values. Between the depths equivalent to 2 to 6 mass units (g/cm 2 ), there is a linear relation between age and integrated mass (Fig. 9), consistent with an assumption that reservoir age and mass accumulation rate at these sites remained constant through this interval. In both cores, these intervals were fit with a regression to determine mass accumulation rate for the studied time period (depth ranges 7-16 cm in MUC9 and 7-14 cm in MUC10). Calculations of radiocarbon sedimentation rates for MUC 9 and 10 yield values of 9.0 and 12.0 mg/cm 2 -yr, respectively, spanning an interval of about 400 years between 250 2 and 6 mass units. This calculation excluded samples in the upper 2 integrated mass units due to apparent bomb 14 C contamination, as both cores show a much younger value of ∆ 14 C in the upper 1 cm of sediments relative to the profile below this depth.. Below the zone that was fitted, ∆ 14 C (BP)* values for MUC 10 were quite erratic, due to several turbidites that were noted in this core. Turbidite influence is also evident in the d 13 Corg profiles (Fig. 10), introducing carbon that is isotopically lighter than the material immediately above and below. All 14 C values below 6 integrated mass units were deemed to have 255 turbidite influence. and were also excluded from the fit.

Excess 210 Pb as a measure of sedimentation rate 260
210 Pb has proven to be a useful tracer for sediment accumulation rates in the Santa Monica Basin (Bruland et al., 1974;Huh et al., 1989;Christensen et al., 1993) and similar environments (Souza et al., 2012) during the last 100 years. Past studies derived mass accumulation rates (MAR) rates using 210 Pb by assuming a constant sedimentary flux of 210 Pb over the time scale concerned (~100 years), negligible bioturbation, and strong absorption of 210 Pb to particles (constant initial concentration 265 method; Benninger and Krishnaswami, 1981;Robbins and Edington, 1975;Robbins, 1978;Appleby, 2001;Oldfield and Appleby, 1984). These assumptions should be valid in the deepest parts of the SMB where sediments are minimally disturbed by bioturbation as shown by laminations. Table 2 shows a compendium of mass accumulation rates for the central portion of SMB, obtained from cores collected during 270 a 42-year interval from 1974 to 2016. MAR values were taken directly from Bruland et al. (1974) Huh et al. (1989, and Christensen et al. (1993), and all studies accounted for sediment compaction. All cores collected from depths >900 m showed MARs that were remarkably consistent, averaging 17.1±0.6 mg/cm 2 -yr (± 1 SDOM). There was also no noticeable trend in MAR (Fig. S3) or variation in the amount of excess 210 Pb at the sediment-water interface over time. Additionally, excess 210 Pb profiles were similar in structure downcore. All cores (Fig. 11), except for those obtained in the present study, were retrieved 275 by box corers, which can disturb the top few centimeters of sediments (Huh et al., 1989). Yet all the cores collected from the deep basin showed remarkable consistency, with no evidence of sedimentation rate change between the 1970's and 2016, as well as no evidence of core disturbance.
We also compared 210 Pb profiles in cores retrieved from water depths 870-900 m (Fig. 12) to those collected from deeper sites 280 as to determine if a trend exists with water depth. These cores showed the same MAR as the deeper sites, although with more variation evidenced in the larger standard deviation of the mean (17.9±1.9 mg/cm 2 -yr). However, as with the deepest cores, we observed no systematic change as a function of year collected (Fig. S3).Much of the variability in MAR was driven by CaBS X BC2, which was collected at 870 m. Core CaBS V BC8 had a clear 210 Pb minimum in the upper 10 cm and featured a 'typical' 210 Pb profile only below this depth. The minimum and the offset of the extrapolated fit for the deeper points from 285 the surface values suggest rapid input of material with low excess 210 Pb, most likely from a localized turbidite in this core. The eight cores collected from 870-900 meters showed surface excess 210 Pb that were similar to cores collected from sites >900 m.
Although cores from the shallower depth range averaged the same MAR as the deeper cores, the quality of the linear fit of excess 210 Pb versus integrated mass, as demonstrated by the average R 2 value, was poorer for cores 870-900 m (average R 2 = 0.90) compared to cores collected at depths of >900 m (average R 2 =0.99 ), suggesting either episodic input of sediment with 290 varying excess 210 Pb or possibly minor episodic disturbances. Christensen et al. (1993) and Huh et al. (1989) documented the concentric areal expansion of laminated sediments throughout 295 the floor of SMB starting about 400 years B.P. Both studies determined that the onset of anoxia began in the south-east portion of the central basin, where the basin is deepest (> 900 m) and moved outward, asymmetrically, but in all directions (Fig. 13).

Changes in the areal extent of laminated sediments
Using the presence of fine laminations as a proxy for oxygen deficiency and establishing the onset of lamination by assignment of age, a 'lateral' anoxic spreading rate of 50-80 m yr -1 was calculated (Christensen et al., 1993). Depending on the direction chosen, the rate of anoxia spreading in vertical space varied, from 0.06 m/year up the eastern slope to 0.19 m/year moving in 300 an NNW direction (Fig. 13). This asymmetry may be attributed to the major circulation pattern of deep basin water, in which waters from the San Pedro Basin enter SMB from the south-east and travel counter-clockwise. In such a flow, the eastern slope of the SMB would be bathed by overlying waters with slightly more oxygen than waters on the north-north-west side of the 10 basin. The overall expansion of anoxic waters may reflect both a reduction of oxygen in waters entering the basin, as well as increased oxygen consumption within deep basin waters. The latter could arise from either an increase rain-rate of labile 305 carbon, or a reduction in water replacement rates.
Only two of the 2016 cores analyzed in the present study showed sedimentary layering in x-radiographs (MUC 9 and MUC 10). The other cores from this study (near the SMB slope) had no laminations and were likely influenced by mixing. For the deepest core, MUC 9, there was clear evidence of finely-laminated sediments in the top 15 cm (Fig. 6). MUC 10, which is 310 located in the southern SMB, near the connection to San Pedro Basin, showed a banding (1-2 cm width) of sediments in the upper few cm of the core but no fine lamination, suggesting minimal bioturbation. Given MUC 10's location in relation to the spread of oxygen deficiency, the absence of finely-laminated sediments in the upper few cm suggests that the spread of oxygen deficiency has not extended to this location. Furthermore, MUC 3 (777 m), which is right at the boundary of the zone of oxygen deficiency defined by Christensen et al. (1993), had no indications of laminations, and 210 Pb clearly showed a mixed 315 zone in the upper 4 cm (Fig. 7). These two MUC cores make it tempting to suggest the oxygen deficiency zone is contracting; however, we can conclude with confidence that the position of the laminated zone in SMB has not changed markedly since cores were last obtained and analyzed in the 1980's.

Changes in mass accumulation rates-A comparison of 210 Pb and 14 C methods 320
Interpretation of 210 Pb and 14 C profiles in terms of sediment accumulation rate rely on assumptions that the delivery of these radio-tracers have been consistent and continuous, and that the sediment has not been disturbed via mixing (typically bioturbation). The assumption of consistency is generally assumed to be true in basins that receive sediments via hemipelagic sedimentation, and the assumption of non-disturbance is supported by sediment fabric as revealed by x-radiography (Fig. 6). 325 The similarity of 210 Pb profiles in core MUC 9, which shows fine lamination structure in the top 8 cm, and core MUC 10, which shows coarser sediment banding, is evidence that some minor disturbance in the latter core may have obscured lamination structure, but disturbance has been insufficient to change the 210 Pb profile. Both of these cores yield similar sediment accumulation rates, ~17 mg/cm 2 -yr, and show no evidence for a change in sedimentation rate over the lifetime of 210 Pb, which 330 is approximately 80-100 years. Because the Bruland (1974) core does not show any evidence of a change in sedimentation rate through the life of 210 Pb, we can conclude that sedimentation has been constant in SMB since around the late 1800s to early 1900s. We find it striking that sediment accumulation offshore from an urban center has remained constant, even though the region has grown from a small town to the present 15+ million-person megalopolis of Los Angeles.

335
While accumulation rates remained constant during the period of rapid population growth in Los Angeles, the 14 C accumulation rates, not including those horizons that lie within turbidite deposits, define a sedimentation rate for the period ~ about 1500-11 1900 C.E. that is less than that defined by 210 Pb. In both MUCs 9 and 10 cores, 14 C dated sediment horizons yield sediment accumulation rates of 9-12 mg/cm 2 -yr compared to 17.1 ± 0.6 derived from 210 Pb profiles (Table 2). This trend of increasing sedimentation toward the recent is opposite of what might have been predicted due to the trapping of sediment via flood-340 control engineering of the LA River. However, our data are consistent with the proposal made by Tomašových and Kidwell (2017) noting that sometime in the mid-late 1800s sediment delivery to the coastal zone of the SMB increased. Tomašových and Kidwell (2017) based their interpretation on the change in the SMB shelf ecosystem structure that occurred at that time.
From the loss of a filter-feeding ecosystem from the SMB shelf environments, Tomašových and Kidwell (2017) infer an increase in fine sediment delivery to the SMB shelf. 345 The determinations of 14 C sediment accumulation rates could be biased or incorrect if there has been a changing input of particulate organic matter (PO 14 C) to the SMB. In MUC 10 there is an obvious section of core where 14 C age dates are old and d 13 Corg values are light, relative to the trend defined by the other data. However, these two measurements are from a turbidite deposit (Fig. 10) and are consistent with the interpretation that such a deposit contains older, terrestrially derived (perhaps 350 more refractory) POC (Meyers, 1994). MUC9 may also show a minor influence from this turbidite, but the effect is subtle. A plot of d 13 Corg versus integrated mass of MUC 9 and MUC 10 show a trend to slightly lighter d 13 Corg near the top, although the change is very small. (Fig 10). The 14 C profiles for both cores appear to show slightly older sediments than expected, just above the 2 mass unit horizon, suggesting a change to additional input of older carbon associated with the modest change in d 13 Corg. The changing trend could record a terrestrial source, but the data are not clear-cut. While there may have been a change 355 in the source of carbon (and sediment) in the late 1800s, the data prior to this time indicates there has been a step-function change in sediment accumulation rate, taking place sometime between the late 1800's to the early 1900's A sensitivity calculation assuming a step-change reduction of 40% in accumulation rate in 1930 (two half-lives before the Bruland (1974) core) shows 210 Pb has the sensitivity to resolve such a change (computed profile not shown). Consequently, the change in accumulation rate must have occurred prior to 1930. 360 It is tempting to suggest that changes in carbon reservoir age or the age of waters upwelling in this region, instead of sedimentation rate, could explain the offset of 14 C values down core. However, if the sedimentation rate determined from the excess 210 Pb profile at MUC 9 is assumed constant down-core to a depth represented by 2-6 mass units, then 236 years would 365 have elapsed during this interval (4 g/cm 2 /0.017 g/cm 2 -yr=236 yr). If the 210 Pb MAR applies through this interval, and the 14 C values record changes in reservoir age and not sedimentation rate, the age for organic carbon (fixed at the surface ocean from DIC) would need to increase by 160 years, at a steady rate, over the time period represented by 2-6 mass units. While this cannot be dismissed, it would imply a higher upwelling rate in the past, and there seems no evidence for this.

12
Another explanation for the lack of 14 C MAR and 210 Pb MAR agreement is that there was a higher proportion of old terrestrial carbon reaching the sediments during the past. However, the lack of a significant change in d 13 Corg through this interval makes this process an unlikely explanation. We think it most likely that an increase in MAR occurred somewhere in the late 1800's and propose that further 14 C analysis of laminated sediments, preserved under low oxygen conditions, is the best way to find further support for this conclusion. 375 A previous study that considered Holocene sediment accumulation in SMB (Romans et al., 2009) found that the hemipelagic sediment accumulation rate for the late Holocene averaged ~10 mg/cm 2 -yr (this rate determined from their linear sediment accumulation rate and the extrapolation of our porosity data to a depth of 2 m), although the turbidite accumulation rate was substantially greater. This is a value consistent with the MAR we found from the 14 C dated section of our cores, only 150-300 380 years before present. Thus, it appears that hemi-pelagic sedimentation in SMB has been very consistent over the past 1000's of years, buthas increased by ~70% through a stepwise change about 100-150 years ago.

385
Only three cores analyzed for this study had macrofauna present, these were MUC 12 (508 m), MUC 8 (695 m) and MUC 11 (745 m). All three cores were collected from bottom waters with <20 µM oxygen concentration and the deeper two sites have <10 µM oxygen. The living annelid found in MUC 11 is evidence that macrofauna can be active and hence potentially act to bioturbate at low oxygen levels (<5 µM).

390
A preponderance of sponge spicules was found in replicate cores from the location of MUC 8. This is also a site bathed in waters with <5 µM oxygen. In addition to the core sectioned for biological inspection, a core that was x-radiographed shows the presence of a partially articulated demosponge within the sediment column at ~8 cm depth (Morine, 2017). These sponges are not infaunal, thus the most plausible explanation for the high spicule abundance in these cores is that this sediment zone has been populated by sponges for >100 years. 395 Prior to the work of Christensen et al. (1993), Malouta et al. (1981) mapped out the area of bioturbation throughout the SMB using x-radiographs of basin cores. Using disturbances in laminated sediments as a proxy for different levels of bioturbation, 3 different zones were assigned: completely disturbed laminae, partially disturbed laminae, and fine laminae present.
Completely disturbed laminae were cores that showed no laminations or banding and were usually found on the shelf and 400 slopes of the SMB, typically shallower than 750m. Partially disturbed laminae were characterized by some hints of banding and suggested minimal bioturbation. Lastly, finely laminated sediments were zones of no bioturbation and were located in the deep, central basin at depths greater than 900m. The areas to which Malouta et al. (1981) assigned these zones of bioturbation correlate with our cores obtained in 2016, suggesting minimal changes in organism activity vs. depth during the last 40 years.
Additionally, our work shows that laminae can be largely obscured and yet a 210 Pb profile from a slightly bioturbated core 405 (MUC 10) can appear nearly indistinguishable from a profile from a well-laminated core (MUC 9).

Conclusions
A suite of cores was collected in 2016 to explore whether changes in the areal extent of laminated sediments and their mass 410 accumulation rates have changed during recent decades. Only one core analyzed in 2016 showed finely laminated sediments in x-radiographs (MUC 9 at 907 m). Other cores showed cm-scale layering of sediments or no layering at all. The absence of finely laminated sediments in MUC 10 (893 m) and MUC 3 (777 m) suggest that the rate of oxygen deficiency-spreading, as noted by Huh et al. (1989) and Christensen et al. (1993) has not increased remarkably since cores were last collected in the 1980's. It is possible that the rate of anoxic bottom water spreading has declined or even possibly reversed with a slightly 415 shrinking area of laminated sediments. X-radiographs of laminations from cores collected in this study were compared to the different levels of bioturbation mapped out 40 years ago in the SMB. The zones of bioturbation correlate with cores collected in 2016, again suggesting minimal change in macrofaunal activity (assumed a proxy for bottom oxygen concentrations) during the last 40 years.

420
Through a summary of previously published profiles and new measurements of 210 Pb in sediment cores from this study, a comparison of mass accumulation rate records in the central portion of SMB was examined in cores collected over a 42 year span. Mass accumulation rates for the deepest parts of the SMB basin (>900m) have been remarkably consistent since the late 1800s, averaging 17.1 ± 0.6 mg/cm 2 -yr. At slightly shallower sites (870-900m), accumulation rates showed a little more variability, but yield the same accumulation rate, averaging 17.9 ± 1.9 mg/cm 2 -yr. Excess 210 Pb near the sediment-water 425 interface was also consistent for all cores deeper than 870 m during the last 4 decades. The consistency of sedimentation rates, both for the past 40 years but also for the lifetime of 210 Pb, ~100 years, is remarkable given the changes that have occurred in the Los Angeles region over the past century.
∆ 14 C values between 7 and 20 cm depths suggest sediment accumulation rates were lower prior to the late 1800s. MUC 9 and 430 MUC 10 reveal sedimentation rates of 8.6 and 12.0 mg/cm 2 -yr prior to the late 1800s, which is 55-75% of the rates determined for younger sediments using the excess 210 Pb profiles. The slower accumulation rate for hemipelagic sediments also occurred during the late Holocene (Roman et al., 2009). The increase in MAR appears to be a step-function change, although the precision of the dating methods can only constrain the transition to somewhere between about 1850 and 1920. A possible explanation, offered by Tomašových and Kidwell (2017), is that sedimentation increased between 1850-1900 due to the rapid 435 rise of cattle grazing and increased erosion. Why this increased rate remained high after urban development, and why it should have remained so constant subsequently, are unanswered questions, particularly following installation of debris basins that trapped a large portion of the sediment flux. Perhaps these basins largely captured coarse debris, while the fine sediment 14 fraction that contributes to hemi-pelagic input has not been captured, but its input was augmented by cattle grazing and subsequent urban development. 440 Evidence of sedimentary change in the SMB during the last 40 years is astonishingly absent. Mass accumulation rates, laminated sediments, extent of bioturbation, and % Corg have changed little during this time. The only parameter that appears to have clearly changed in the last 200 years is the sedimentation rate, which shows a step-function increase in the late 1800s to early 1900s.     Table 2). Note that the depth and activity scales are slightly different for different cores.   Table 2). Arrows on the y-axis indicate the integrated mass equivalence to the year 1900 CE. Figure 12: Same as Fig. 12 but for 8 cores obtained from depths between 870-900 meters.