Evidence of changes in sedimentation rate and sediment fabric in a low-oxygen setting: Santa Monica Basin, CA
<p>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 discharge, pollution, flood control measures, and global warming. Santa...
| Main Authors: | , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-04-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/17/2381/2020/bg-17-2381-2020.pdf |
| Summary: | <p>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
discharge, pollution, flood control measures, and global warming. Santa
Monica Basin (SMB), due to its unique setting in a low-oxygen and high-sedimentation environment, can provide an excellent sedimentary paleorecord
of these anthropogenic changes. This study examined 10 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.</p>
<p>The <span class="inline-formula"><sup>210</sup>Pb</span>-based mass accumulation rates (MARs) for the deepest and lowest oxygen-containing parts of the SMB basin (900–910 m) have been remarkably
consistent during the past century, averaging <span class="inline-formula">17.1±0.6</span> mg cm<span class="inline-formula"><sup>−2</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>. At slightly shallower sites (870–900 m), accumulation rates
showed more variation but yield the same accumulation rate, <span class="inline-formula">17.9±1.9</span> mg cm<span class="inline-formula"><sup>−2</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>. Excess <span class="inline-formula"><sup>210</sup>Pb</span> sedimentation rates were consistent
with rates established using bomb test <span class="inline-formula"><sup>137</sup>Cs</span> profiles. We also examined
<span class="inline-formula"><sup>14</sup>C</span> profiles from two cores collected in the deepest part of the SMB,
where fine laminations are present up to about 450 yr BP. These data
indicate that the MAR was slower prior to <span class="inline-formula">∼1900</span> CE (rates
obtained were 9 and 12 mg cm<span class="inline-formula"><sup>−2</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>). The <span class="inline-formula"><i>δ</i><sup>13</sup>C<sub>org</sub></span> profiles
show a relatively constant value where laminations are present, 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.</p>
<p>X-radiographs show finely laminated sediments in the deepest part of the
basin only, with centimeter-scale layering of sediments or no layering whatsoever in
shallower parts of the SMB basin. The absence of finely laminated sediments
in cores MUC 10 (893 m) and MUC 3 (777 m) suggests 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 have changed minimally during the last
40 years. Specifically, mass accumulation rates, laminated sediment fabric,
extent of bioturbation and % <span class="inline-formula">C<sub>org</sub></span> have not changed. The only
parameter that appeared to have changed in the last 450 years was the MAR,
with an apparent > 50 % increase occurring between <span class="inline-formula">∼</span>1850 CE and the early 1900s. The post-1900 CE constancy of
sedimentation through a period of massive urbanization in Los Angeles is
surprising.</p> |
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| ISSN: | 1726-4170 1726-4189 |