Reproducibility of Ba/Ca variations recorded by northeast Pacific bamboo corals

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 32 (2017): 966–979, doi:10.1002/2017PA003178.Trace elemental ratios preserved in the calcitic skeleton of bamboo corals have been shown to serve as archives of past ocean conditions. The concentration of dissolved barium (BaSW), a bioactive nutrientlike element, is linked to biogeochemical processes such as the cycling and export of nutrients. Recent work has calibrated bamboo coral Ba/Ca, a new BaSW proxy, using corals spanning the oxygen minimum zone beneath the California Current System. However, it was previously unclear whether Ba/Cacoral records were internally reproducible. Here we investigate the accuracy of using laser ablation inductively coupled plasma mass spectrometry for Ba/Cacoral analyses and test the internal reproducibility of Ba/Ca among replicate radial transects in the calcite of nine bamboo corals collected from the Gulf of Alaska (643–720 m) and the California margin (870–2054 m). Data from replicate Ba/Ca transects were aligned using visible growth bands to account for nonconcentric growth; smoothed data were reproducible within ~4% for eight corals (n = 3 radii/coral). This intracoral reproducibility further validates using bamboo coral Ba/Ca for BaSW reconstructions. Sections of the Ba/Ca records that were potentially influenced by noncarbonate bound Ba phases occurred in regions where elevated Mg/Ca or Pb/Ca and coincided with anomalous regions on photomicrographs. After removing these regions of the records, increased Ba/Cacoral variability was evident in corals between ~800 and 1500 m. These findings support additional proxy validation to understand BaSW variability on interannual timescales, which could lead to new insights into deep sea biogeochemistry over the past several centuries.NSF Grant Number: OCE-1420984; NOAA/OE Grant Number: NA16RP2637; MIT-WHOI Joint Program; American Geophysical Union Travel Grant; UC Davis President's Undergraduate Fellowship; Bowdoin College Gibbons Summer Research Fellowship2018-03-1

Coincident Lake Drainage and Grounding Line Retreat at Engelhardt Subglacial Lake, West Antarctica

Antarctica has an active subglacial hydrological system, with interconnected subglacial lakes fed by subglacial meltwater. Subglacial hydrology can influence basal sliding, inject freshwater into the sub-ice-shelf cavity, and impact sediment transport and deposition which can affect the stability of grounding lines (GLs). We used satellite altimetry data from the ICESat, ICESat-2, and CryoSat-2 missions to document the second recorded drainage of Engelhardt Subglacial Lake (SLE), which began in July 2021 and discharged more than 2.3 km3 of subglacial water into the Ross Ice Shelf cavity. We used differential synthetic aperture radar interferometry from RADARSAT-2 and TerraSAR-X alongside ICESat-2 repeat-track laser altimetry (RTLA) and REMA digital elevation model strips to detect 2–13 km of GL retreat since the previous drainage event in 2003–06. Combining these satellite observations, we evaluated the mechanism triggering SLE drainage, the cause of the observed GL retreat, and the interplay between subglacial hydrology and GL dynamics. We find that: (a) SLE drainage was initiated by influx from a newly identified upstream lake; (b) the observed GL retreat is mainly driven by the continued retreat of Engelhardt Ice Ridge and long-term dynamic thinning that caused a grounded ice plain to reach flotation; and (c) SLE drainage and GL retreat were largely independent. We also discuss the possible origins and influence of a 27 km grounded promontory found to protrude seaward from the GL. Our observations demonstrate the importance of high-resolution satellite data for improving the process-based understanding of dynamic and complex regions around the Antarctic Ice Sheet margins

Nitrogen isotopes of amino acids in marine sediment: A burgeoning tool to assess organic matter quality and changes in supplied nitrate δ15N

Compound-specific nitrogen (N) isotopic analysis (δ15N) of amino acids (AA), or CSI-AA, is a novel approach to understand N cycling. We expand upon initial observations at a productive, hypoxic margin to provide insight into source and transformation of sedimentary organic nitrogen from varied depositional conditions in a complex N cycling zone, the eastern tropical Pacific. δ15NAA patterns are generally well preserved, matching original mixed plankton inputs with some evidence for microbial degradation. There is appreciable δ15N offset between total N (bulk) and total AA N, where AA N is enriched. Source AA, previously understood to record changing baseline δ15N, are fundamentally different in marine sediments. We use modeling to rigorously test whether the total AA δ15N instead tracks changing baseline δ15N. Lastly we employ this new understanding of δ15NAA in sediments to compare with published whole sediment (δ15Nbulk) records, demonstrating the utility and wealth of information provided by CSI-AA

Carbon and Nitrogen Isoscapes in West Antarctica Reflect Oceanographic Transitions

Antarctic marine ecosystems are spatially and temporally dynamic. Regional climate change is significantly altering the patterns and magnitudes of this dynamism with cascading impacts on biogeochemistry, productivity, and food web architecture. Isoscapes (or isotopic maps) provide a valuable analytical framework to characterize ecosystem processes and address questions about trophic dynamics, animal movement, and elemental cycling. Applications of stable isotope methods to Antarctic ecosystems are currently limited by a paucity of information on geospatial isotope characteristics within the Southern Ocean. In response, we have created the first empirically derived zooplankton isoscapes for West Antarctica based on analysis of bulk nitrogen and carbon isotope values (δ15N and δ13C) in 94 zooplankton specimens from the Drake Passage, West Antarctic Peninsula (WAP), and Amundsen and Ross Seas. The zooplankton δ15N values increased by 3‰ from north of the Polar Front (3.3 ± 0.6‰, mean ± SD) to the Ross Sea (6.2 ± 0.8‰), reflecting a productivity gradient across this region. Abundant open water polynyas in the Amundsen and Ross Seas exhibit strong nitrate drawdown, resulting in more 15N-enriched phytoplankton and zooplankton relative to those from the generally less productive WAP and Drake Passage. Zooplankton δ13C values decreased by 3‰ from north of the Polar Front (-24.2 ± 0.9‰) to the Ross Sea (-27.5 ± 1.6‰), likely driven by decreasing sea surface temperatures with increasing latitude. Our isoscapes are a valuable first step in establishing isotopic spatial patterns in West Antarctica and are critical for addressing numerous ecosystem questions

Paleoceanographic Insights on Recent Oxygen Minimum Zone Expansion: Lessons for Modern Oceanography

Climate-driven Oxygen Minimum Zone (OMZ) expansions in the geologic record provide an opportunity to characterize the spatial and temporal scales ofOMZ change. Here we investigate OMZ expansion through the global- scale warming event of the most recent deglaciation (18- 11 ka), an event with clear relevance to understanding modern anthropogenic climate change. Deglacial marine sediment records were compiled to quantify the vertical extent, intensity, surface area and volume impingements of hypoxic waters upon continental margins. By integrating sediment records (183-2,309 meters below sea level; mbsl) containing one or more geochemical, sedimentary or microfossil oxygenation proxies integrated with analyses of eustatic sea level rise, we reconstruct the timing, depth and intensity of seafloor hypoxia. The maximum vertical OMZ extent during the deglaciation was variable by region: Subarctic Pacific (similar to 600-2,900 mbsl), California Current (similar to 330-1,500 mbsl), Mexico Margin (similar to 330-830 mbsl), and the Humboldt Current and Equatorial Pacific (similar to 110-3,100 mbsl). The timing ofOMZ expansion is regionally coherent but not globally synchronous. Subarctic Pacific and California Current continental margins exhibit tight correlation to the oscillations of Northern Hemisphere deglacial events (Termination IA, Bolling-Allerod, Younger Dryas and Termination IB). Southern regions (Mexico Margin and the Equatorial Pacific and Humboldt Current) exhibit hypoxia expansion prior to Termination IA (similar to 14.7 ka), and no regional oxygenation oscillations. Our analyses provide new evidence for the geographically and vertically extensive expansion of OMZs, and the extreme compression of upper-ocean oxygenated ecosystems during the geologically recent deglaciation

Subarctic Pacific (SP) deglacial core data synthesized into hypoxia categories.

<p>Changing deglacial core depths reflect global eustatic sea level change. The encircled number adjacent to each core label corresponds to the number of available oxygenation proxies, which are enumerate in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115246#pone.0115246.t003" target="_blank">Table 3</a>. Vertical grey bars correlate to temporal intervals in OMZ geospatial reconstructions for this region.</p

Schemata of a multi-proxy approach to interpreting hypoxia categories, including severe hypoxia ([O₂]<0.5 ml L^-1), intermediate hypoxia ([O₂]>0.5–1.5 ml L^-1) and mild hypoxia to oxic conditions ([O₂]>1.5 ml L^-1).

<p>These hypoxia categories are detailed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115246#pone.0115246.t001" target="_blank">Table 1</a>, and follow <i>Hofmann et al.</i>, [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115246#pone.0115246.ref024" target="_blank">24</a>]. Hypoxia proxies include [Re], [Mn], [U], [Cd], [Mo], δ¹⁵N, foraminiferan communities, and sedimentary laminations. Units for each proxy reflect the cited literature, which constrains the proxy to a specific oxygenation category.</p

Seafloor hypoxia proxies for paleoceanographic reconstructions, partitioned by the thresholds and capacity each proxy has to record fine-scale changes in seafloor hypoxia, as well as organic flux to the seafloor [24].

<p>Seafloor hypoxia proxies for paleoceanographic reconstructions, partitioned by the thresholds and capacity each proxy has to record fine-scale changes in seafloor hypoxia, as well as organic flux to the seafloor [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115246#pone.0115246.ref024" target="_blank">24</a>].</p

Global Oxygen Minimum Zones, including (a) Upper depth (in meters) of intermediate water hypoxia ([O₂]<1.4 ml L^-1) and (b) thickness (in meters) of intermediate water hypoxia ([O₂]<1.4 ml L^-1).

<p>The geospatial distributions of severely hypoxic [O₂] minimums (of [O₂] = 0.5 ml L^-1 and [O₂] = 0.2 ml L^-1) are depicted on both panels as white lines. For the upper panel, regional blocks are defined by black lines to highlight where paleoxygenation reconstructions were completed. Data from World Ocean Atlas [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115246#pone.0115246.ref192" target="_blank">192</a>].</p