Stable isotopic composition of deep-sea gorgonian corals Primnoa spp.: a new archive of surface processes

The deep-sea gorgonian coral Primnoa spp. lives in the Atlantic and Pacific Oceans at depths of 65-3200 m. This coral has an arborescent growth form with a skeletal axis composed of annual rings made from calcite and gorgonin. It has a lifespan of at least several hundred years. It has been suggested that isotopic profiles from the gorgonin fraction of the skeleton could be used to reconstruct long-term, annual-scale variations in surface productivity. We tested assumptions about the trophic level, intra-colony isotopic reproducibility, and preservation of isotopic signatures in a suite of modern and fossil specimens. Measurements of gorgonin {Delta}{sup 14}C and {delta}{sup 15}N indicate that Primnoa spp. feed mainly on zooplankton and/or sinking particulate organic matter (POM{sub SINK}), and not on suspended POM (POM{sub SUSP}) or dissolved organic carbon (DOC). Gorgonin {delta}{sup 13}C and {delta}{sup 15}N in specimens from NE Pacific shelf waters, NW Atlantic slope waters, the Sea of Japan, and a South Pacific (Southern Ocean sector) seamount were strongly correlated with Levitus 1994 surface apparent oxygen utilization (AOU; the best available measure of surface productivity), demonstrating coupling between skeletal isotopic ratios and biophysical processes in surface water. Time-series isotopic profiles from different sections along the same colony were identical for {delta}{sup 13}C, while {delta}{sup 15}N profiles became more dissimilar with increasing separation along the colony axis. Similarity in C:N, {delta}{sup 13}C and {delta}{sup 15}N between modern and fossil specimens suggest that isotopic signatures are preserved over millennial timescales. Finally, the utility of this new archive was demonstrated by reconstruction of 20th century bomb radiocarbon

Pathways and transformations of dissolved methane and dissolved inorganic carbon in Arctic tundra watersheds: Evidence from analysis of stable isotopes

Arctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO2) and methane (CH4) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem-scale CH4 and CO2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO2) and CH4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH4 and DIC balance by estimating mechanisms of CH4 and DIC production and transport pathways and oxidation of subsurface CH4. We observed a shift from acetoclastic (July) toward hydrogenotropic (September) methanogenesis at sites located toward the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape-scale effect on CH4 production mechanism. The majority of subsurface CH4 was transported upward by plant-mediated transport and ebullition, predominantly bypassing the potential for CH4 oxidation. Thus, surprisingly, CH4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH4 produced at the frost table, contributing to <0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e- acceptors. This work highlights the importance of spatial and temporal variability of CH4 production at the watershed scale and suggests broad scale investigations are required to build better regional or pan-Arctic representations of CH4 and CO2 production

Microstructure, growth banding and age determination of a primnoid gorgonian skeleton (Octocorallia) from the late Younger Dryas to earliest Holocene of the Bay of Biscay

A fossil primnoid gorgonian skeleton (Octocorallia) was recovered on the eastern Galician Massif in the Bay of Biscay (NE Atlantic) from 720 m water depth. The skeleton shows a growth banding of alternating Mg–calcitic and organic (gorgonin) increments in the inner part, surrounded by a ring of massive fibrous calcite. Three calcite-dominated cycles, bounded by thick organic layers, consist of five light-dark couplets of calcite and gorgonin. Two AMS-14C datings of the fossil skeleton give ages of 10,880 and 10,820 ± 45 14C years before present (BP). We arrive at a calibrated age range of 11,829–10,072 cal. years BP (two σ), which comprises the late Younger Dryas to the earliest part of the Holocene. The cyclic calcitic–organic growth banding may be controlled by a constant rate of calcite secretion with a fluctuating rate of gorgonin production, possibly related to productivity cycles. The skeletal fabric change of alternating calcitic–organic increments to massive fibrous calcite may be the result of hydrographic changes during the deglaciation as reflected by preliminary stable isotope data. If this hypothesis proves to be correct, primnoid gorgonians are able to match with varying hydrodynamic conditions by changing their biomineralisation mode

Skeletal morphogenesis and growth mode of modern and fossil deep-water isidid gorgonians (Octocorallia) in the West Pacific (New Zealand and Sea of Okhotsk)

Fabric and growth mode of deep-water isidid gorgonian skeletons showing bright Mg-calcitic internodes and dark proteinageous nodes were investigated on modern, subrecent and fossil skeletons. The internodial microstructure is characterised by three-dimensionally interfingering calcitic fascicles accreting around a central axis. Macroscopic colour banding results from varying orientations of organic-rich fascicle bundles and intercalated bands of organic-poor granular crystals. This skeletal structure of isidid gorgonians strikingly differs from the density banding of scleractinians. Radiocarbon dating of a fossil skeleton gave an age of 3,985±35 to 3,680±35 years before present (BP) with a record of 305±35 years (±range). Linear extension rates of 0.4 mm year−1 average allow for an annual to sub-annual resolution on micrometer scale of colour bands or fascicles, respectively. The growth mode of branched skeletons is characterised by simultaneous secretion of vertically alternating nodes/internodes and lateral accretion of concentric increments enveloping the entire skeleton. Bifurcations at various growth stages imply that adjacent branches have different ages and show varying numbers of growth bands at any skeletal cross section. The scleroprotein gorgonin plays a crucial role in the formation of organic nodes and the secretion of calcitic internodes by providing a structural framework in the biomineralisation process