Description: PP53B-1204 Instrumental and proxy data and global climate model experiments indicate a multi-decadal shoaling of the western tropical Pacific (WTP) thermocline potentially related to a shift in ENSO frequency. In the WTP, the nutricline coincides with the thermocline, and a shoaling of the nutricline brings more nitrate-rich seawater higher in the water column and within the sunlit euphotic zone. In the nutrient-poor WTP, this incursion of nitrate-rich water at the bottom of the euphotic zone may stimulate productivity in the water column. However, there is a general paucity of measurements below the surface with which to investigate recent changes in seawater chemistry. Nitrogen isotope (δ15N) measurements of particulate organic matter (POM) can elucidate the source of nitrogen to the WTP and related trophic dynamics. This POM is the food source to the long-lived proteinaceous corals, and drives the nitrogen isotopic composition of their skeleton. Here, we report time series δ15N values from the banded skeletons of proteinaceous corals from offshore Palau in the WTP that provide proxy information about past changes in euphotic zone nitrogen dynamics. Bulk skeletal δ15N values declined between 1977 and 2010 suggesting a progressively increasing contribution of deep water with isotopically-light nitrate to the euphotic zone and/or a shortening of the planktonic food web. Since only some amino acids are enriched in δ15N with each trophic transfer in a food web, we measured the δ15N composition of seven individual amino acids in the same coral skeleton. The δ15N time series of the individual amino acids also declined over time, mirroring the bulk values. These new data indicate that the changes in the source nitrogen to the base of the euphotic zone drives a decline in coral skeletal δ15N values, consistent with the shoaling nutricline, with no coinciding alteration of the trophic structure in the WTP.

Description: The ram’s horn squid, Spirula spirula (Spirulida, Coleoidea), inhabits subsurface waters of the tropical and subtropical oceans. Because of the presence of an internal chambered shell in this species, it has frequently been used as a model species to investigate the paleobiology of fossil coleoids. However, the feeding and dietary habits of S. spirula in the nature are poorly known. In this study, we applied a new method (amino acid nitrogen isotopic analysis) to estimate the trophic position of S. spirula specimens captured off Suriname, as well as three cuttlefish Sepia species (Sepia officinalis, S. latimanus, and S. esculenta), with a calcified chambered shell from the shallower water. The trophic position of S. spirula was estimated to be 2.5–2.8, which was significantly lower than that for the three Sepia species (3.4–3.6). The results and available data on the gastric contents of S. spirula suggest that it feeds mainly on detritus and zooplankton, including crustaceans, from the overlying water column. The method used in this study can potentially be applied to the estimation of the trophic position of the fossil cephalopods having calcified chambered shells.

Description: We have investigated the distributions and carbon isotopic compositions of archaeal membrane lipids in gas-hydrate-bearing sediments collected from the northern Cascadia Margin offshore from Vancouver Island (Sites U1327 and U1328) by the R/V JOIDES Resolution during IODP Expedition 311. Archaeal lipid biomarkers, including glycerol dialkyl glycerol tetraethers (GDGTs), tend to become abundant below 100 mbsf (meters below sea floor). Tricyclic biphytane (BP[3]; which is a robust biomarker derived from GDGT), crenarchaeol, and other BPs exhibit d13C values of ca. -20 per mil, and become abundant between 130 and 230 mbsf at Site U1328. In this depth range, concentrations of ammonium and phosphate in interstitial waters also increase, suggesting that a larger population and higher activity of heterotrophic community consisting of crenarchaeota and other archaea decompose the sedimentary organic matter, thereby liberating ammonium and phosphate. Such crenarchaeotic activity can produce other metabolic products such as molecular hydrogen by fermentation of organic matter during diagenesis. Furthermore, near the organic matter decomposition zone (130 to 230 mbsf), a probable methanogen biomarker (13C-depleted BP[1] with d13C values as low as -48.8 per mil) becomes abundant, indicating that methanogens utilize these diagenetic products. The molecular and isotopic distributions of archaeal lipid biomarkers indicate that the archaeal community plays an important role in the biogeochemical cycles of deep-sea sediments, including both methanogenesis and nutrient recycling.

Description: δ15N of abundant chlorins (pheophytin a) and bulk material from leaves, litter, and soil across four sites of a precipitation gradient on leeward Kohala volcano, Hawaii.

Description: A sediment core collected from Caroline Basin, western tropical Pacific was analyzed for lipid class compounds (aliphatic hydrocarbons, long-chain alkenones, fatty alcohols, sterols, and fatty acids) to reconstruct changes in paleoenvironments during the last 23.3 kyr. Around the boundary between the glacial and deglacial periods (19 ka), mass accumulation rates of terrestrial biomarkers, C25-C35 n-alkanes, C24-C28 fatty alcohols, and C23-C34 fatty acids, were found to decrease significantly and stayed in low levels during the deglaciation, suggesting a reduction of atmospheric transport of continental materials during that time. In the same period, mass accumulation rates of C17-C20 n-alkanes, pristane, cholesterol, and dinosterol which are thought to be mainly derived from marine organisms also decreased, suggesting a significant drop of marine biological productivity. The decreased biological productivity in the western tropical Pacific may be caused by a reduced supply of nutrients from upwelling which is associated with decreased wind velocity and/or caused by a shift of upwelling zone.

Description: Chloropigments and their derivative pheopigments preserved in sediments can directly be linked to photosynthesis. Their carbon and nitrogen stable isotopic compositions have been shown to be a good recorder of recent and past surface ocean environmental conditions tracing the carbon and nitrogen sources and dominant assimilation processes of the phytoplanktonic community. In this study we report results from combined compound-specific radiocarbon and stable carbon and nitrogen isotope analysis to examine the time-scales of synthesis and fate of chlorophyll-a and its degradation products pheophytin-a, pyropheophytin-a, and 132,173-cyclopheophorbide-a-enol until burial in Black Sea core-top sediments. The pigments are mainly of marine phytoplanktonic origin as implied by their stable isotopic compositions. Pigment ?15N values indicate nitrate as the major uptake substrate but 15N-depletion towards the open marine setting indicates either contribution from N2-fixation or direct uptake of ammonium from deeper waters. Radiocarbon concentrations translate into minimum and maximum pigment ages of approximately 40 to 1200 years. This implies that protective mechanisms against decomposition such as association with minerals, storage in deltaic anoxic environments, or eutrophication-induced hypoxia and light limitation are much more efficient than previously thought. Moreover, seasonal variations of nutrient source, growth period, and habitat and their associated isotopic variability are likely at least as strong as long-term trends. Combined triple isotope analysis of sedimentary chlorophyll and its primary derivatives is a powerful tool to delineate biogeochemical and diagenetic processes in the surface water and sediments, and to assess their precise time-scales.