PP43A-1504
Insights into Nitrogen Isotopic Fractionation During Algal Assimilation of Nitrate and Ammonium
Nitrogen availability is an important factor controlling algal growth in marine environments, representing a limiting nutrient throughout much of the global ocean. Anthropogenic inputs to the coastal zone may shift the nutrient regime, leading to questions regarding the extent of anthropogenic nutrient impacts in near-shore environments. A large body of work has been completed relating the δ15N of algae, seagrasses, and other benthic organisms to anthropogenic nutrient sources. However, previous work by our research group characterizing the δ15N of organic material associated with waste water discharge points, and in reef and embayment environments of the south Florida coastal zone, has suggested that δ15N values alone do not provide unequivocal evidence of anthropogenic nitrogen loading. Greater understanding of nitrogen processing and isotopic fractionation in coastal benthic organisms is necessary before blanket assumptions regarding nutrient uptake and source association can be universally accepted. Closed system mesocosm incubations examining fractionation associated with assimilation of nitrate and ammonium in cultured red algae, Gracilaria sp. and Agardhiella sp., were completed under varied nitrate and ammonium concentrations from 10 to 500 μM with initial nitrogen isotopic compositions of 2.7-3 ‰. Following 8-day incubations, the isotopic composition of new algal growth ranged between +2.43 and -5.77 ‰, with more depleted values coincident with higher N-availability. Rayleigh fractionation calculations yield fractionation factors of 4-9 ‰ (α values of 1.0045 to 1.008), which represent significantly larger values than those previously reported in the literature for macroalgae. 15N-tracer experiments (initial δ15N = 1000 ‰) were also conducted to assess nutrient preferences in the cultured algae. Isotopic composition of new algal growth varied from -1.3 to +495.0 ‰ with only Agardhiella exhibiting an obvious preference for ammonium. Gracilaria may have a competitive advantage as a generalist in terms of nutrient acquisition. As a consequence of the large assimilation factors and species-specific nutrient preferences, significant variations in the stable nitrogen isotopic composition can be produced by simply varying concentrations of available nitrogen. These results cast doubt on conclusions in previous studies attributing isotopic enrichment in algal tissue to anthropogenic nutrient inputs (sewage) to coastal waters.
PP43A-1505
Modeling Nitrogen Isotopes in the Global Ocean
The nitrogen isotopic signal measured in marine sediments has the potential to be a valuable paleoceanographic proxy. It captures the response of different biological processes in the marine ecosystem including photosynthesis, nitrogen fixation, denitrification as well as processes within the food chain. A simple marine ecosystem model that includes the interactive cycling of nitrogen, phosphorus, and oxygen is augmented to record nitrogen isotopes in the University of Victoria Earth System Climate Model. New nitrogen isotopic tracers are employed at all trophic levels of the ecosystem. This includes the δ15N of nitrate, both classes of phytoplankton (nitrogen fixers and all other phytoplankton), zooplankton, and detritus. Despite a few shortcomings, it is shown that the nitrogen isotope model can capture the major trends observed in the modern climate. The ability to model nitrogen isotopes in a global coupled ocean- atmosphere-sea ice-ecosystem model gives us a unique opportunity to directly infer what physical and biological changes in the climate system are driving the δ15N signal on spatial and temporal scales. This is a valuable tool giving us tremendous insight on how to interpret the nitrogen isotopic signal.
PP43A-1506
The Impact of Emerging Forest Ecosystems on the Marine Nitrogen Cycle in the Middle to Late Devonian
Before the emergence of extensive lowland forest ecosystems in the late Middle Devonian, the flux of reactive nitrogen (both organic and inorganic forms) from the land surface via riverine and atmospheric transport to epeiric and coastal seas was small. Driven by eustatic sea level rise, the expanding areal extent of Devonian forests generated an increasing outwelling flux of organic matter and reactive nitrogen from rivers and estuaries that resulted in elevated primary productivity and episodic high organic content black shale deposition. Nitrogen and carbon stable isotope analyses of sediments at or near the Frasnian/Famennian boundary within the Appalachian Basin indicate that utilization of abundant NO3- was low and that terrestrial organic matter was plentiful during episodes of high primary productivity corresponding to the globally correlated Upper and Lower Kellwasser horizons. High Corg:P values from similarly-aged sediments suggest that the supply of P necessary to support high productivity was remobilized from organic matter within oxygen-depleted sediments. Thus, a feedback loop initiated by sea level rise and fueled by terrestrially-derived fixed N served to increase marine productivity, organic matter deposition, benthic dysoxia, and P remobilization. The feedback mechanism was terminated by a regressive episode that diluted organic matter deposition with an increasing sediment flux. The biotic impact of elevated productivity among benthic invertebrates may have included increased population sizes and consequent diminished rates of origination, accounting for a significant proportion of Middle to Late Devonian biodiversity loss.
PP43A-1507
Dissolved N2/Ar Ratios in Sedimentary Pore Waters: A New Twist in Marine Nitrogen Cycle
The nitrogen cycle is comprised predominantly of biologically mediated pathways, leading to a series of negative feedbacks that stabilize the cycle. Sedimentary denitrification, the major sink in the nitrogen budget, is regulated by the rate of organic carbon rain to the sea floor, as well as oxygen concentrations in overlying bottom waters. The sensitivity of sedimentary denitrification as a negative feedback can be expressed as a ratio between total denitrification (including nitrification sub-cycle) rates integrated over depth (fluxes) and fluxes of remineralized organic carbon out of the sediments, Ndenitr/Coxid_total. We have investigated benthic nitrogen cycling in three, semi-enclosed basins of the California Borderlands: Santa Monica, San Pedro and Santa Barbara located in the regime of seasonal coastal upwelling. Deep water in these basins is separated from the open ocean by sills of various depths, contributing to the low [O2], <1 to10 uM. In this study, we developed a method to sample pore waters for dissolved gas analysis. Ratios between O2, Ar and N2 were determined on extracted pore waters with 1) offline cryogenic extraction and subsequent analysis on Finnigan Delta Plus IRMS with 8 collectors; 2) Membrane Inlet Mass Spectrometery (MIMS). Vertical profiles of pore water N2/Ar in the three basins indicate N2 production at depth horizons which exceed by a factor of 5 to 20 the depth of nitrate penetration supported solely by diffusive flux. At depths of maximum subsurface N2 production, we discovered large pools of intracellular nitrate. The relationship between δ15N and δ18O of nitrate are consistent with the activity of a membrane-bound nitrate reductase affecting the measured isotopic composition of the nitrate pool (Granger et al., 2008, in press). In addition, increases in δ15N of pore water NH4 at this depth suggests that at least some of the nitrate might be used for anaerobic ammonium oxidation. Our model estimates up to 25 % of the measured total nitrate flux into the sediments must be transported by non-diffusive processes to support the subsurface N2 production rate. We hypothesize that nitrate is transported to depth by motile microorganisms, bacteria or protists, for use in yet to be identified reactions with sulfide, dissolved metals or organic matter. We propose that the non-local transport of nitrate also sustains the population of Anammox performing bacteria, living in close association with nitrate transporting organisms and contributing to the subsurface N2 production. This process presents yet another pathway for nitrate losses from the oceans, increasing sensitivity of sedimentary denitrification as a stabilizing feedback in the nitrogen cycle, as the conversion of transported nitrate to N2 may not be directly linked to oxidation of the organic carbon.
PP43A-1508
Epiphyte Density and Diversity on Halimeda incrassata and the Affect on Bulk Isotopic Measurements
Epiphytic density and diversity, organic and inorganic δ13C and organic δ15N were determined for the green calcifying benthic macroalgae, Halimeda incrassata, from Biscayne Bay, a subtropical coastal lagoon located off the southeastern coast of Florida. Cocconeis and Bacteriastrum were determined to be the two most dominant diatomaceous epiphytes living on the Halimeda tissue but an unidentified bacteria proved to be the overall most abundant epiphyte. Cocconeis and the bacteria reached maximum densities mid-strand while Bacteriastrum reached a bimodal peak density at the top and bottom of the strand. Organic δ13C are consistently lighter towards the top of the strand while inorganic δ13C shows the opposite pattern. These results indicate that photosynthetic rates are higher at the top of the strand where metabolic processes are able to preferentially uptake 12C . δ13C values can vary as much as 3 ‰ by algal segment and inorganic values tend to approach equilibrium with the surrounding seawater in the lower algal segments, which are the oldest parts of the thallus. Organic δ15N shows no definite independent trend but increases in δ15N appear to correlate with increased density of the Cocconeis diatom. Organic δ15N can vary as much 9 ‰ by algal segment. This work questions the validity of using bulk isotopic measurements of the algae as a proxy for the origin of nitrogen. Future work should address quantification of epiphytic and Halimeda sp. biomass, the possible presence of other bacteria within the algal tissue and skeletal structure, the relationship between the bacteria and algal host (symbiotic vs. parasitic), and determine how epiphytic communities vary over time (seasons) and space (location within the Bay).
PP43A-1509
Diffuse Reflectance Spectroscopy as a Qualitative Indicator of Paleopigment Concentrations in Lake Sediment
In natural waters, the presence of chlorophyll a (Chl a) and its isomer and degradation products (commonly referred to as CDP) results in a ubiquitous absorption feature in the visible-near infrared red portion of the electromagnetic spectrum (660-720 nm). Recent studies have identified this feature in lacustrine sedimentary profiles and have correlated its relative magnitude to analytically-derived CDP and Chl a concentrations using visible-near infrared diffuse reflectance spectroscopy (VNIRS). Here, we utilize VNIRS to determine sedimentary CDP and Chl a concentrations in lakes from the Baroon Taiga Mountains of northern Mongolia and the North Island of New Zealand. This study further tests the potential of VNIRS as a rapid, inexpensive, and non-destructive method of qualitative sedimentary paleopigment concentration analysis and as a possible proxy for lake productivity reconstructions. Direct regression of VNIRS-derived data with CDP + Chl a concentration values determined by high potential liquid chromatography and spectrophotometry in sediment cores from lakes Sanjin Nuur and Ganbold Nuur, northern Mongolia, provide high and significant coefficients of correlation (r2 > 0.8, p < 0.0001). Specific factors resulting from principle component analysis of the spectral data provide less significant but similarly acceptable correlation to CDP + Chl a values (r2 > 0.3, p < 0.01). Application of the VNIRS method to multiple sediment profiles replicates previously determined biogenic silica and organic matter-inferred paleoproductivity records from several regional lake systems. Visible-near infrared reflectance spectroscopy of sediments from a lake of dissimilar geographic, ecological, and physical characteristics (Lake Pupuke, New Zealand) provides an additional test of the method's validity. The resultant Lake Pupuke VNIRS-inferred CDP and Chl a profile is compared to associated organic matter concentration and C/N ratio records to initially assess its value as a paleoproductivity indicator.
PP43A-1510
Nitrogen stable isotope fractionation along trophic level pathways in deep sea bamboo corals
Deep-sea bamboo corals (order Gorgonacea, family Isididae) found on seamounts on the California Margin (37° 22'N – 31° 54'N) have the potential to record annual to subannual changes in water mass chemistry over long periods of time (75-300 years). These corals are composed of a two-part skeleton of calcite internodes segmented by proteinaceous organic nodes. Radiocarbon analyses of the organic nodes show the presence of the anthropogenic bomb spike, reflecting a surface-derived food source for the corals (pre-bomb values of -81.3 per mil - -116. per mil, post-bomb values of 61.8 per mil - 81.3 per mil). Nitrogen stable isotopic analyses of the organic nodes (average δ15N = 13.78 per mil – 18.30 per mil) suggest that the corals feed on surface-derived organic matter, although not directly on sinking particulate organic matter but rather on an intermediary such as zooplankton. This study examines the nitrogen stable isotopic content of surface water plankton, bamboo coral polyp material, and bamboo coral organic node material sampled along the California Margin in 2007. Comparing these data with previously established zooplankton nitrogen stable isotope data will enable investigation of the pathway of surface water organic sources to bamboo coral organic material.
PP43A-1511
Million year cycles in the Fe, Mg and Ni records of a ferromanganese crust from the equatorial Indian Ocean
In search of long term productivity signals, a high resolution geochemical study was undertaken by using the
life sustaining iron and magnesium contents in a slowly accreting 26 mm thick hydrogenous Fe-Mn crust
representing around 12 Million years (Ma) record from the equatorial Indian Ocean. We analyzed Fe, Mg, Ni,
Co, and other trace metals by using electron probe micro-analyzer at 100 micron interval. The geochemical
data was averaged at every 1 mm interval and subjected to statistical analyses. The crust was dated using
standard cobalt-chronometry (Manheim and Lane-Bostwick, 1998). Mixed age-depth model (Heegaard et al.,
2005) was applied to ascertain the error limits in the computed ages for each millimeter of the crust.
Thereafter, the Red-fit (Schulz and Mudelsee, 2002) and multi-taper (Thompson, 1990) spectral analyses of
Fe, Mg and Ni revealed the existence of the significant (>90%) cycles at around 3, 1.5, and 1.2 Ma. We
surmise that Fe and Mg cycles represented the changes in oceanic productivity as these metals are
essentially used in sustaining the oceanic phyto- and zoo-plankton productivity in the surface water. The
Fe/Ni ratio, which is attributed to meteoritic dust influx (Johnson, 2001), also revealed the similar cycles
suggesting a possibility of Ni input from the meteoritic dust in the past. We compared the geochemical time-
series data with the Earth's orbital eccentricity and summer solar insolation (Berger, 1979) at the equator for
the last 10 million years. The Redfit and multi-taper analyses of the eccentricity and the insolation also
resulted similar cycles at around 1.5 and 1.2 Ma. Therefore, we surmise that the Fe, Mg, and Ni cycles at 1.5,
and 1.2 Ma could be result of the geochemical response to the Earth's eccentricity related solar insolation
changes. Earlier studies reported cycles due to eccentricity (0.4, 0.126, 0.95 Ma), tilt (0.041 Ma) and
precession (0.023 Ma) in Indian Ocean, whereas we report here 3, 1.5 and 1.2 Ma supra-Milankovitch cycles
for the first time from the equatorial Indian Ocean.
http://www.nio.org
PP43A-1512
Iron oxides, dissolved silica, and regulation of marine phosphate concentration
Phosphorous concentrations in iron oxide-rich sediments reflect orthophosphate levels in the water column from which iron oxides precipitated. Sediment P/Fe ratios are also strongly influenced by the concentrations of dissolved species that inhibit orthophosphate-to-ferrihydrite sorption, most notably silica. It may, therefore, be possible to use P/Fe ratios in iron oxide-rich sediments to estimate past dissolved P concentrations, if one considers the evolution of the silica cycle. A compilation of Fe and P data in iron oxide-rich sediments through time reveals an increase in P/Fe ratios after the Jurassic. We propose that this trend indicates evolution of the iron-oxide phosphate removal mechanism caused by decreasing levels of sorption inhibition by dissolved silica. The large difference in P/Fe ratios in Cenozoic versus older iron-oxide rich sediments can be linked with Si drawdown caused by the proliferation of siliceous plankton in the Cretaceous. There is also a late Mesozoic or Cenozoic increase in V/Fe ratios, which provides additional evidence for lower ferrihydrite anion sorption efficiency prior to diatom radiation. P/Fe ratios in iron oxide-rich sediments from the early and middle Phanerozoic are comparable to the ratios in iron formations previously presented as evidence for an early Precambrian phosphate crisis (Bjerrum and Canfield, 2002, Nature, 417:159-162). Given the compelling evidence for higher Si concentrations in the Precambrian compared to the Phanerozoic and dissolved P concentrations comparable to modern levels throughout the Phanerozoic, the presented trend of P/Fe ratios suggests dissolved P concentrations were higher in Precambrian than Phanerozoic oceans. High dissolved P levels in the Precambrian may have been linked to inhibited carbonate fluorapatite (CFA) formation as a result of persistently high levels of carbonate supersaturation. Carbonate ion substitution into CFA scales with the ambient carbonate ion activity and increases the solubility of the CFA. Substitution of the carbonate ion into the fluorapatite structure, therefore, results in an exponential increase in the levels of dissolved P needed for CFA precipitation. Elevated dissolved P concentrations in Precambrian oceans likely resulted in nitrogen-limited primary production on geological time scales.
PP43A-1513
Pre-Snowball Earth ecosystems, insights from nitrogen isotopes in the Neoproterozoic Kwagunt Formation of the Chuar Supergroup, Grand Canyon.
Reconstructing marine ecosystems prior to the Neoproterozoic Snowball Earth episodes is an important constraint to our understanding of these events and to the biogeochemical evolution of the Neoproterozoic. The mixed siliciclastic and carbonate sequence comprising the Chuar Supergroup, Grand Canyon, USA, encompasses the period in Earth history immediately prior to the first of the Snowball Earth episodes 740 million years ago and presents a unique opportunity explore this issue in relatively immature rocks. In the Phanerozoic, there is a consistent relationship between euxinic basins, widespread black shale deposition, and δ15N values below 0‰ that are indicative of nitrogen-fixation supported primary productivity. Fe-speciation data from black shales of the Walcott Member of the Kwagunt Formation, upper Chuar Supergroup suggest that the Chuar water column and the middle Neoproterozoic deep ocean was euxinic. Bulk δ15N values from the Walcott Member range from +1.7 to +4.7‰, which are not directly supportive of nitrogen fixation. Preliminary nitrogen isotope data from organic extracts suggest that organic nitrogen may be more significantly enriched than bulk isotopes suggest. These relatively 15N- enriched values contrast those from Phanerozoic episodes of black shale deposition. The Walcott primary producer community is interpreted to have utilized a nitrogen substrate that had an isotopic composition more like that of nitrate in modern, relatively oxygenated marine systems. On the basis of biomarker data many black shale sequences of the Phanerozoic (e.g. Devonian, Permo-Triassic and Cretaceous) record the presence of an active phototrophic sulfide oxidizer community. In contrast, our best efforts have yet to yield biomarker evidence of photic zone euxinia within the Walcott. We therefore suggest that the sulfidic chemocline during Walcott deposition was consistently below the photic zone. Oxygenic phototrophic primary producers utilized nitrate from the region above the sulfidic zone that is 15N-enriched due to water- column denitrification under dysoxic conditions. Stratigraphic maxima in total organic carbon (10.8%) and δ15N values (+4.7‰) co-occur and sedimentological parameters indicate basin deepening and reduced siliciclastic dilution as the primary factor in total organic carbon enrichments. Higher δ15N values during basin deepening may reflect a more dysoxic upper water column and a greater influence of denitrification on the δ15N of dissolved inorganic nitrate utilized by primary producers.
PP43A-1514
The Nitrogen Cycle in the Ocean of the Late Archean
We have measured the nitrogen isotope composition of kerogen extracted from minimally altered, organic-rich shales formed in the late Archean (2,700 to 2,400 Ma) from a AGPDP core drilled on the Campbellrand-Malmani platform in South Africa. Average δ15N values are +2.9 ±1‰ and range from 2 to 5 ‰. The range in kerogen δ15N increases to 0.5 to 7.3 ‰ if samples associated with volcanic tuffs or tuff zones are included. Our data fall on a trend of increasing δ15N which starts from close to 0 ‰ in the early Archean and reaches 5 ‰ by the middle Paleoproterozoic. The small increase in δ15N we measure at 2670 Ma suggests some loss of 14 from the ocean had already occurred. The majority of the core was deposited in relatively shallow water. We find there is little change in kerogen δ15N through most of the core which suggests that the N-cycle leading up to the end of the Archean was stable, i.e., no large losses or gains of N. There is a moderate increase of 3 ‰ in kerogen δ15N in sediments deposited between 2520 and 2460 Ma as water depths increased prior to the deposition of the Kuruman BIF. With an anoxic ocean interior and a partially oxidized surface resulting from oxygenic photosynthetic activity, the strongest redox gradient is likely to occur at the based of the mixed layer. If light penetrates beyond the surface mixed layer, photosynthetic organisms can use surface N which is most sensitive to nitrification-denitrification and can have higher δ15N values than deep NH4+, the second source of N. The increase in kerogen δ15N with increasing water depth when the NH4+ becomes more available is compatible with the N-cycle increasingly influenced by O, and losing 14N. The subsequent return to lower δ15N could indicate N- limitation of oxygenic photosynthetic organisms. Alternatively the influence of O-cycle on the N-cycle could have been restricted by Fe(II) competition for O. Large epeiric seas covering the Kaapvaal craton between 2600 and 2400 Ma may have allowed oxygenic photosynthesis to occur without the presence of deep Fe(II) reaching the surface layer. Platform drowning may have allowed ferruginous seawater to reach and scavenge O from the surface layer lowering rates of nitrification-denitrification, re-emphasizing the reductive part of the N-cycle. How changes in oxygen production and or O-buffering by redox sensitive chemicals influences the N-cycle warrants closer scrutiny.
PP43A-1515
Redox Control of N:P Ratios in Aquatic Ecosystems
The nitrogen cycle, like those of carbon and sulfur, is primarily controlled by the redox state of the environment. As a result, the oxygen content of the water column plays a critical role in the speciation and distribution of nitrogen compounds. In contrast, phosphorus input is primarily controlled by input from rock weathering via river runoff. It has been generally thought that the ratio of dissolved fixed inorganic nitrogen to soluble inorganic phosphate (N:P) in a water body was determined by the elemental ratio of the remineralized organic matter. While the atomic N:P ratio in the ocean interior has been determined to be relatively constant at ~ 16:1, the elemental ratio for lakes and other aquatic environments spans more than six orders of magnitude. The relationship of nitrogen cycling to oxygen concentration led us to question whether the N:P ratio is solely controlled by organic matter decomposition, or whether other characteristics of a water body play a role. In order to understand the factors influencing N:P ratios in a variety of aquatic environments, we analyzed 104 deep water observational data sets obtained from 33 water bodies, ranging from small lakes to ocean basins. Our results reveal that when oxygen concentrations are below ~ 100 μM, the N:P ratios are highly correlated with the concentration of dissolved O2. At higher O2 concentrations, the correlation ceases, and N:P ratios become highly variable. In all cases, the normalized variance in observed N:P ratios is strongly linked to the size of the water body. Hence, classical Redfield ratios observed in the ocean are anomalous; observed N:P ratios result not only as a consequence of the elemental ratio of the sinking flux of organic matter, but also are correlated to the size of the basins and their ventilation. As a result, it may be possible to use the link between N:P ratios, basin size, and oxygen levels, along with the previously determined relationship between sedimentary δ15N and oxygen, to infer historical N:P ratios for any water body.