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1. OXYGEN AND CARBON ISOTOPES IN ADAMUSSIUM COLBECKI: δ13C PROXY FOR SEA-ICE PERSISTENCE?

   https://doi.org/doi: 10.1130/abs/2020AM-357341  

   Cronin, K. ; Walker, S.E. ; Gillikin, D.P. ; Bowser, S.S. ( October 2020 , Geological Society of America Abstracts with Programs)

   null (Ed.)

   The Antarctic scallop Adamussium colbecki is a promising proxy for sea-ice persistence and can potentially resolve subannual seawater conditions characteristic of annual and multiannual sea ice. Alternating groups of widely- and narrowly-spaced striae (small ridges on valve surfaces) are thought to indicate seasonal growth differences: wide groups in summer, narrow groups in winter. Shell oxygen (δ18Os) and carbon (δ13Cs) in striae groups may therefore reflect seasonal seawater conditions. We expect lower δ18Os in wide summer striae groups under both annual and multiannual sea ice if glacial meltwater mixes through the water column. We also expect higher δ13Cs in wide striae groups under annual sea ice but not under multiannual sea ice, as phytoplankton blooms post seaice breakout enrich seawater δ13CDIC. Scallops were collected from two sites in western McMurdo Sound (Ross Sea) located ~30 km apart: Explorers Cove (EC) has multiannual sea ice and Bay of Sails (BOS) has annual sea ice. Adults were collected live by divers at 9–18 m depth in 2008 from EC and BOS. Additional juveniles (< 2 yrs) were collected from EC in 2016. Two adults each from EC and BOS and two 2016 juveniles were serially sampled for stable isotopes. δ13Cs decreases over ontogeny due to metabolic effects; the linear trend was removed to enable seasonal comparison. Detrended residuals are referred to as δ13Cs det. Mean δ18Os (~3.7‰) is not different in narrow and wide striae groups under either annual or multiannual sea ice, suggesting negligible glacial meltwater mixing at depth and minimal seasonal temperature change at both sites. δ18Os values are within expected equilibrium range and decrease over ontogeny, suggesting increased growth during warmer temperatures in older scallops. In contrast, mean δ13Cs det is ~1‰ higher in wide summer striae groups than narrow winter striae groups under annual sea ice at BOS, but not different between striae groups under multiannual sea ice in EC adults. δ13Cs det is also higher in wide summer striae groups from 2016 EC juveniles, however sea ice broke out at EC in 2015, so juveniles experienced annual-like sea-ice conditions. Seasonal differences in δ13Cs suggest that carbon isotopes coupled with striae width in A. colbecki may be a good proxy for sea-ice persistence in Antarctica both in modern and fossil assemblages. 

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2. THE ANTARCTIC SCALLOP ADAMUSSIUM COLBECKI AS A PROXY FOR SEA-ICE DURATION IN ANTARCTICA

   https://doi.org/10.1130/abs/2019AM-340824  

   CRONIN, K. E. ; WALKER, S. E. ; GILLIKIN, D. P. ; BOWSER, S. S. ( October 2019 , Geological Society of America Abstract with Programs)

   null (Ed.)

   Sea ice is critical in structuring Antarctic marine ecosystems, controlling disturbance and primary productivity. Sea ice either melts annually or persists for multiple years, but variability in sea-ice duration is poorly understood prior to satellite images. The Antarctic scallop Adamussium colbecki, with its circum-Antarctic distribution and Holocene fossil history, may be a proxy for sea-ice duration. Previous work on A. colbecki links some trace elements to ice melt and productivity. Further, increments between growth bands (striae) are thought to vary seasonally. To evaluate A. colbecki suitability as a sea-ice proxy, we tested correspondence between growth and trace elements known to represent sea ice or productivity at two sites in western McMurdo Sound: Explorers Cove (EC) with multiannual sea ice and Bay of Sails (BOS) with annual sea ice. Trace element signals should be dampened or absent at EC, whereas those from BOS should cycle annually. One A. colbecki shell each from EC and BOS were collected live in 12 m of water. Trace elements previously linked to ice melt (Mn/Ca, Fe/Ca, and Pb/Ca), metabolism (Mg/Ca), and primary productivity (Ba/Ca, Li/Ca) were sampled from interstrial increments using an LA-ICP-MS along the central axis from umbo to last striae. Interstrial distances (ISDs) were measured and compared to trace elements using wavelet coherence analysis. Coherence (covariance between ISD and trace elements) exceeding 95% significance are reported here. Results indicate that ISD and trace elements only cohere during episodic sea-ice melt at EC and cohere throughout adult growth at BOS. All EC trace element concentrations display a common pattern: cyclic growth followed minimal variation in early adult ontogeny, with intermittent variation resuming later in adult growth. In contrast, trace elements from the BOS scallop exhibit strong cyclic behavior throughout ontogeny. ISD coheres with trace elements at EC for short strial sequences (5-30) twice in adult growth, corresponding to partial sea ice melts at EC during 1999 and 2002. Conversely, BOS trace elements cohere with ISD for long (20-140) strial sequences during adult growth, indicating annual sea-ice melt. Results indicate that A. colbecki archives sea-ice duration, thus its fossil record can be used to investigate past variability. 

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3. TRACE ELEMENTS AND INTERSTRIAL DISTANCES AS ENVIRONMENTAL AND ANTHROPOGENIC PROXIES IN THE ANTARCTIC SCALLOP, ADAMUSSIUM COLBECKI

   CRONIN, K. ; WALKER, S.E. ; GILLIKIN, D. ; BOWSER, S. ( July 2019 , PaleoBios)

   null (Ed.)

   The Antarctic scallop Adamussium colbecki may be a crucial paleoenvironmental proxy for coastal Antarctica. For example, two highly seasonal environmental parameters, glacial melt and productivity, were linked to trace elemental concentrations in a previous bulk shell analysis and a transect spanning ~ 3 months of juvenile growth. However, neither study examined seasonal variation in trace elements or tied variation to distances between small ridges (striae) on valve surfaces, which may also vary seasonally. Striae and interstrial growth between them are expressed as alternating narrow and wide groups (presumably winter and summer growth, respectively). If tied to trace elemental concentrations, striae could provide high-resolution sclerochronological proxies for seawater conditions. Here, we evaluate whether trace elements archived in A. colbecki striae can be used as seasonal indicators of glacial influence and nutrients over A. colbecki ontogeny. We examined trace elements from an adult and juvenile A. colbecki (shell height, 80.2 mm and 17.1 mm, respectively) collected live by divers from ~ 12 m water depth in Explorers Cove, western McMurdo Sound (2008 and 2016, respectively). Trace elements linked to glacial melt (Mg/Ca, Mn/Ca, Fe/Ca, and Pb/ Ca), metabolism (Mg/Ca), and productivity (Ba/Ca) were sampled with an LA-ICP-MS on each stria along the central growth axis of lower (right) valves from umbo to growing margin. Distances between sampled striae were measured along the central margin (FIJI). Interstrial distances (ISDs) and trace elements were compared using wavelet coherence analysis (Wavelet- Comp 1.1) and cross-correlation. Coherence and correlations that exceeded 95% significance are reported here. Coherence identifies areas of covariance between ISD and trace elements over ontogeny; cross-correlation describes the direction (±) of correlation between 113 NAPC 2019 PROGRAM & ABSTRACTS ISDs and trace elements where coherence exists. We expected trace elements that increase with glacial melt (Fe, Mn, Pb), productivity (Ba), and altered metabolism (Mg) to be coherent and correlate positively with ISD (highest concentrations at wide summer striae) throughout ontogeny. Preliminary results mostly do not conform to predictions. Though correlation remains consistently positive or negative under strong coherence, most elements are only coherent with ISD for short strial sequences (~ 8 striae) and only during adult growth. Of the elements associated with glacial melt, only Mn correlates positively with ISD and may be a potential proxy for seasonality. Other indicators of glacial melt (Pb/Ca, Fe/Ca) and productivity (Ba/Ca) correlate negatively with ISD. Mg/Ca correlates positively with ISD, indicating seasonal effects on metabolism. Ontogenetic variation in coherence urges cautious use of ISDs as proxies, but Pb/Ca (anthropogenic in Antarctica) is coherent with ISD throughout ontogeny; further analysis might illuminate seasonal effects of human activities on Antarctic ecosystems. 

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4. MOLLUSK FORENSICS IN ANTARCTICA: DO EPI- BENTHIC SCALLOPS EXHIBIT PREDATORY SHELL REPAIR?

   WALKER, S. ; BOWSER, S.S. ( June 2019 , PaleoBios)

   null (Ed.)

   One of the most fundamental changes predicted to occur under warming scenarios for Antarctica is the invasion of durophagous (shell-breaking or peeling) predators—like decapod crustaceans—which were last common in Antarctic waters during the warmer Eocene Period, over 30 million years ago. Since then, Antarctica’s shallow-water benthos developed Paleo- zoic (or deep-sea-like) ecosystems dominated by epi- benthic echinoderms. Despite the looming predatory carnage, little is known about how predators structure shallow subtidal communities in Antarctica, especially in regard to predation on shelled prey. We therefore need to have a baseline of shell repair—if it occurs— prior to the initial invasion of crabs. Here, we assess whether the shell of the Antarctic Scallop, Adamussium colbecki, living in the shallow subtidal under sea ice, records an ontogenetic history of shell repair. Shells of A. colbecki(n=623 valves; ~ 0.50 mm thick) were collected from shallow depths (6–24 m) within western McMurdo Sound, Ross Sea, from the coldest waters on Earth (-1.97 °C): Four sites in Explorers Cove (EC) with semi-permanent (decadal or more) sea ice and a Ferrar Glacier site (located ~30 km south of EC) with annual sea ice and icebergs. All sites were composed of fine sediments interspersed with glacial erratics that were more common at Ferrar than EC. Ju- venile (≤ 50 mm) and adult portions of the shells were examined under a dissecting scope for shell repair. Results indicate that repair did occur and was consistent with predatory damage: 1) valves had ste- reotypic damage patterns, both in style and spatial distribution; 2) there were five styles of repair rang- ing from typical crab-like (jagged) repair to elongate repair; 3) scallops living under ice scour regimes (Ferrar) did not have significantly different repair frequencies than those living under semi-permanent sea ice (EC sites); and 4) none of the shells had shell repair consistent with ice scour as described previ- ously for Laternula elliptica, an Antarctic burrowing bivalve. Frequency of repair varied between 0.04 and 0.26 for the five sites and depths (mean 0.10) and adults had the highest frequency of repair. The mean repair frequency is similar to infaunal Laternulafrom other semi-permanent sea ice sites in McMurdo Sound, but higher than those reported for epifaunal brachio- pods from the Antarctic Peninsula where ice scour does occur. We posit that shell repair can be used as an indicator of durophagy in Antarctica: The forensic agents are unexpectedly sea stars and possibly fish. In a warming world, this scallop may not survive long withboth an increase in ice scour and the putative ar- rival of shell-breaking crabs at ~1 °C. 

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5. Florida mangrove saltmarsh reference surface soils

   https://doi.org/10.6073/pasta/0e08cbe07c84488cb7b9dd16669946d0  

   Lewis, David Bruce ( January 2021 , Environmental Data Initiative)

   Site description. This data package consists of data obtained from sampling surface soil (the 0-7.6 cm depth profile) in black mangrove (Avicennia germinans) dominated forest and black needlerush (Juncus roemerianus) saltmarsh along the Gulf of Mexico coastline in peninsular west-central Florida, USA. This location has a subtropical climate with mean daily temperatures ranging from 15.4 °C in January to 27.8 °C in August, and annual precipitation of 1336 mm. Precipitation falls as rain primarily between June and September. Tides are semi-diurnal, with 0.57 m median amplitudes during the year preceding sampling (U.S. NOAA National Ocean Service, Clearwater Beach, Florida, station 8726724). Sea-level rise is 4.0 ± 0.6 mm per year (1973-2020 trend, mean ± 95 % confidence interval, NOAA NOS Clearwater Beach station). The A. germinans mangrove zone is either adjacent to water or fringed on the seaward side by a narrow band of red mangrove (Rhizophora mangle). A near-monoculture of J. roemerianus is often adjacent to and immediately landward of the A. germinans zone. The transition from the mangrove to the J. roemerianus zone is variable in our study area. An abrupt edge between closed-canopy mangrove and J. roemerianus monoculture may extend for up to several hundred meters in some locations, while other stretches of ecotone present a gradual transition where smaller, widely spaced trees are interspersed into the herbaceous marsh. Juncus roemerianus then extends landward to a high marsh patchwork of succulent halophytes (including Salicornia bigellovi, Sesuvium sp., and Batis maritima), scattered dwarf mangrove, and salt pans, followed in turn by upland vegetation that includes Pinus sp. and Serenoa repens. Field design and sample collection. We established three study sites spaced at approximately 5 km intervals along the western coastline of the central Florida peninsula. The sites consisted of the Salt Springs (28.3298°, -82.7274°), Energy Marine Center (28.2903°, -82.7278°), and Green Key (28.2530°, -82.7496°) sites on the Gulf of Mexico coastline in Pasco County, Florida, USA. At each site, we established three plot pairs, each consisting of one saltmarsh plot and one mangrove plot. Plots were 50 m^2 in size. Plots pairs within a site were separated by 230-1070 m, and the mangrove and saltmarsh plots composing a pair were 70-170 m apart. All plot pairs consisted of directly adjacent patches of mangrove forest and J. roemerianus saltmarsh, with the mangrove forests exhibiting a closed canopy and a tree architecture (height 4-6 m, crown width 1.5-3 m). Mangrove plots were located at approximately the midpoint between the seaward edge (water-mangrove interface) and landward edge (mangrove-marsh interface) of the mangrove zone. Saltmarsh plots were located 20-25 m away from any mangrove trees and into the J. roemerianus zone (i.e., landward from the mangrove-marsh interface). Plot pairs were coarsely similar in geomorphic setting, as all were located on the Gulf of Mexico coastline, rather than within major sheltering formations like Tampa Bay, and all plot pairs fit the tide-dominated domain of the Woodroffe classification (Woodroffe, 2002, "Coasts: Form, Process and Evolution", Cambridge University Press), given their conspicuous semi-diurnal tides. There was nevertheless some geomorphic variation, as some plot pairs were directly open to the Gulf of Mexico while others sat behind keys and spits or along small tidal creeks. Our use of a plot-pair approach is intended to control for this geomorphic variation. Plot center elevations (cm above mean sea level, NAVD 88) were estimated by overlaying the plot locations determined with a global positioning system (Garmin GPS 60, Olathe, KS, USA) on a LiDAR-derived bare-earth digital elevation model (Dewberry, Inc., 2019). The digital elevation model had a vertical accuracy of ± 10 cm (95 % CI) and a horizontal accuracy of ± 116 cm (95 % CI). Soil samples were collected via coring at low tide in June 2011. From each plot, we collected a composite soil sample consisting of three discrete 5.1 cm diameter soil cores taken at equidistant points to 7.6 cm depth. Cores were taken by tapping a sleeve into the soil until its top was flush with the soil surface, sliding a hand under the core, and lifting it up. Cores were then capped and transferred on ice to our laboratory at the University of South Florida (Tampa, Florida, USA), where they were combined in plastic zipper bags, and homogenized by hand into plot-level composite samples on the day they were collected. A damp soil subsample was immediately taken from each composite sample to initiate 1 y incubations for determination of active C and N (see below). The remainder of each composite sample was then placed in a drying oven (60 °C) for 1 week with frequent mixing of the soil to prevent aggregation and liberate water. Organic wetland soils are sometimes dried at 70 °C, however high drying temperatures can volatilize non-water liquids and oxidize and decompose organic matter, so 50 °C is also a common drying temperature for organic soils (Gardner 1986, "Methods of Soil Analysis: Part 1", Soil Science Society of America); we accordingly chose 60 °C as a compromise between sufficient water removal and avoidance of non-water mass loss. Bulk density was determined as soil dry mass per core volume (adding back the dry mass equivalent of the damp subsample removed prior to drying). Dried subsamples were obtained for determination of soil organic matter (SOM), mineral texture composition, and extractable and total carbon (C) and nitrogen (N) within the following week. Sample analyses. A dried subsample was apportioned from each composite sample to determine SOM as mass loss on ignition at 550 °C for 4 h. After organic matter was removed from soil via ignition, mineral particle size composition was determined using a combination of wet sieving and density separation in 49 mM (3 %) sodium hexametaphosphate ((NaPO_3)_6) following procedures in Kettler et al. (2001, Soil Science Society of America Journal 65, 849-852). The percentage of dry soil mass composed of silt and clay particles (hereafter, fines) was calculated as the mass lost from dispersed mineral soil after sieving (0.053 mm mesh sieve). Fines could have been slightly underestimated if any clay particles were burned off during the preceding ignition of soil. An additional subsample was taken from each composite sample to determine extractable N and organic C concentrations via 0.5 M potassium sulfate (K_2SO_4) extractions. We combined soil and extractant (ratio of 1 g dry soil:5 mL extractant) in plastic bottles, reciprocally shook the slurry for 1 h at 120 rpm, and then gravity filtered it through Fisher G6 (1.6 μm pore size) glass fiber filters, followed by colorimetric detection of nitrite (NO_2^-) + nitrate (NO_3^-) and ammonium (NH_4^+) in the filtrate (Hood Nowotny et al., 2010,Soil Science Society of America Journal 74, 1018-1027) using a microplate spectrophotometer (Biotek Epoch, Winooski, VT, USA). Filtrate was also analyzed for dissolved organic C (referred to hereafter as extractable organic C) and total dissolved N via combustion and oxidation followed by detection of the evolved CO_2 and N oxide gases on a Formacs HT TOC/TN analyzer (Skalar, Breda, The Netherlands). Extractable organic N was then computed as total dissolved N in filtrate minus extractable mineral N (itself the sum of extractable NH_4-N and NO_2-N + NO_3-N). We determined soil total C and N from dried, milled subsamples subjected to elemental analysis (ECS 4010, Costech, Inc., Valencia, CA, USA) at the University of South Florida Stable Isotope Laboratory. Median concentration of inorganic C in unvegetated surface soil at our sites is 0.5 % of soil mass (Anderson, 2019, Univ. of South Florida M.S. thesis via methods in Wang et al., 2011, Environmental Monitoring and Assessment 174, 241-257). Inorganic C concentrations are likely even lower in our samples from under vegetation, where organic matter would dilute the contribution of inorganic C to soil mass. Nevertheless, the presence of a small inorganic C pool in our soils may be counted in the total C values we report. Extractable organic C is necessarily of organic C origin given the method (sparging with HCl) used in detection. Active C and N represent the fractions of organic C and N that are mineralizable by soil microorganisms under aerobic conditions in long-term soil incubations. To quantify active C and N, 60 g of field-moist soil were apportioned from each composite sample, placed in a filtration apparatus, and incubated in the dark at 25 °C and field capacity moisture for 365 d (as in Lewis et al., 2014, Ecosphere 5, art59). Moisture levels were maintained by frequently weighing incubated soil and wetting them up to target mass. Daily CO_2 flux was quantified on 29 occasions at 0.5-3 week intervals during the incubation period (with shorter intervals earlier in the incubation), and these per day flux rates were integrated over the 365 d period to compute an estimate of active C. Observations of per day flux were made by sealing samples overnight in airtight chambers fitted with septa and quantifying headspace CO_2 accumulation by injecting headspace samples (obtained through the septa via needle and syringe) into an infrared gas analyzer (PP Systems EGM 4, Amesbury, MA, USA). To estimate active N, each incubated sample was leached with a C and N free, 35 psu solution containing micronutrients (Nadelhoffer, 1990, Soil Science Society of America Journal 54, 411-415) on 19 occasions at increasing 1-6 week intervals during the 365 d incubation, and then extracted in 0.5 M K_2SO_4 at the end of the incubation in order to remove any residual mineral N. Active N was then quantified as the total mass of mineral N leached and extracted. Mineral N in leached and extracted solutions was detected as NH_4-N and NO_2-N + NO_3-N via colorimetry as above. This incubation technique precludes new C and N inputs and persistently leaches mineral N, forcing microorganisms to meet demand by mineralizing existing pools, and thereby directly assays the potential activity of soil organic C and N pools present at the time of soil sampling. Because this analysis commences with disrupting soil physical structure, it is biased toward higher estimates of active fractions. Calculations. Non-mobile C and N fractions were computed as total C and N concentrations minus the extractable and active fractions of each element. This data package reports surface-soil constituents (moisture, fines, SOM, and C and N pools and fractions) in both gravimetric units (mass constituent / mass soil) and areal units (mass constituent / soil surface area integrated through 7.6 cm soil depth, the depth of sampling). Areal concentrations were computed as X × D × 7.6, where X is the gravimetric concentration of a soil constituent, D is soil bulk density (g dry soil / cm^3), and 7.6 is the sampling depth in cm. 

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