Iron availability limits the ocean nitrogen inventory stabilizing feedbacks between marine denitrification and nitrogen fixation

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 21 (2007): GB2001, doi:10.1029/2006GB002762.Recent upward revisions in key sink/source terms for fixed nitrogen (N) in the oceans imply a short residence time and strong negative feedbacks involving denitrification and N fixation to prevent large swings in the ocean N inventory over timescales of a few centuries. We tested the strength of these feedbacks in a global biogeochemical elemental cycling (BEC) ocean model that includes water column denitrification and an explicit N fixing phytoplankton group. In the northern Indian Ocean and over longer timescales in the tropical Atlantic, we find strong stabilizing feedbacks that minimize changes in marine N inventory over timescales of ∼30–200 years. In these regions high atmospheric dust/iron inputs lead to phosphorus limitation of diazotrophs, and thus a tight link between N fixation and surface water N/P ratios. Maintenance of the oxygen minimum zones in these basins depends on N fixation driven export. The stabilizing feedbacks in other regions are significant but weaker owing to iron limitation of the diazotrophs. Thus Fe limitation appears to restrict the ability of N fixation to compensate for changes in denitrification in the current climate, perhaps leading the oceans to lose fixed N. We suggest that iron is the ultimate limiting nutrient leading to nitrogen being the proximate limiting nutrient over wide regions today. Iron stress was at least partially alleviated during more dusty, glacial times, leading to a higher marine N inventory, increased export production, and perhaps widespread phosphorus limitation of the phytoplankton community. The increased efficiency of the biological pump would have contributed to the glacial drawdown in atmospheric CO2.This work was supported by grants from the U.S. National Science Foundation (OCE-0222033 and OCE-0452972). Computations supported by Earth System Modeling Facility (NSF ATM-0321380) and by the Climate Simulation Laboratory at the National Center for Atmospheric Research

Composition, production, and loss of carbohydrates in subtropical shallow subtidal sandy sediments: rapid processing and long-term retention revealed by 13C-labeling

The composition and production of carbohydrates (mannose, rhamnose, fucose, galactose, glucose, and xylose) and their transfer among sediment compartments (microphytobenthos [MPB], bacteria, and detritus) was investigated through in situ labeling with 13C-bicarbonate. After 60 h, 13C was found in all sediment compartments, demonstrating rapid transfer of fixed carbon from autotrophs to heterotrophs. Carbohydrates were a major carbon reservoir, accounting for 30% (day 0) to 15% (day 30) of the 13C within sediments, and probably played a role in this transfer. Carbohydrate fractions were highly reactive (65-87%), less reactive (7- 18%), and nonreactive (6-23%) over the experimental period. The rate of loss of the less reactive fraction (0.01- 0.05 d-1) was at least an order of magnitude lower than that for the highly reactive fraction (0.8-4.4 d-1). Patterns of diagenesis estimated from label uptake and loss matched the carbohydrate composition observed in the sediment (glucose \u3e galactose \u3e rhamnose \u3e fucose \u3e xylose \u3e mannose) and were similar to patterns reported previously. C:N ratios and δ13C of sediment organic matter indicated an algal origin (MPB and phytoplankton). Although carbon was rapidly processed, loss from sediments was not immediate, and there was evidence of recycling into MPB and bacteria. Rapid transfer of carbon to and from carbohydrates has been found in various environments, including temperate, muddy, and intertidal sediments, and this study demonstrates the important role of carbohydrates in supporting heterotrophic production over extended periods (\u3e 30 d) in subtropical shallow subtidal sands

Nitrogen incorporation and retention by bacteria, algae, and fauna in a sub-tropical intertidal sediment: an in-situ \u3csup\u3e15\u3c/sup\u3eN-labeling study

We performed a 15N-labeling study to investigate nitrogen incorporation and retention by the benthic microbial community (bacteria and benthic microalgae) and fauna in the intertidal sediment of the subtropical Australian Brunswick Estuary. The main experiment involved an in situ 15N pulse–chase experiment. After injection of 15NH4+ into the sediment, 15N was traced into bulk sediment, total hydrolyzable amino acids (THAAs, representing bulk proteinaceous biomass), the bacterial biomarker D-alanine, and fauna over a 30- d period. Additional experiments included short-term (24 h) incubations of sediment cores injected with different 15N-labeled substrates (NH4+, NO3-, urea, and an amino acid mixture) and sediment core incubations for analysis of benthic fluxes of O2, dissolved inorganic carbon, NH4+, NOx-, dissolved organic nitrogen, and N2. 15N was rapidly incorporated and strongly retained in microbial biomass (THAAs) during the 30-d period in situ, indicating efficient recycling of 15N by the benthic microbial community. Analysis of 15N in D-alanine revealed a major bacterial contribution (50–100%) to total microbial 15N incorporation and retention. 15N was also incorporated into fauna via grazing on 15N-labeled microbial biomass, but this was a negligible fraction (15N in the sediment. Altogether, results show that efficient recycling of nitrogen by the benthic microbial community can be an important mechanism for nitrogen retention in the sediment and an important pathway supporting benthic microbial production

Financial Management 3B

Exam paper for second semester Financial Management 3

Seagrass sediments as a global carbon sink: Isotopic constraints

Seagrass meadows are highly productive habitats found along many of the world\u27s coastline, providing important services that support the overall functioning of the coastal zone. The organic carbon that accumulates in seagrass meadows is derived not only from seagrass production but from the trapping of other particles, as the seagrass canopies facilitate sedimentation and reduce resuspension. Here we provide a comprehensive synthesis of the available data to obtain a better understanding of the relative contribution of seagrass and other possible sources of organic matter that accumulate in the sediments of seagrass meadows. The data set includes 219 paired analyses of the carbon isotopic composition of seagrass leaves and sediments from 207 seagrass sites at 88 locations worldwide. Using a three source mixing model and literature values for putative sources, we calculate that the average proportional contribution of seagrass to the surface sediment organic carbon pool is ∼50%. When using the best available estimates of carbon burial rates in seagrass meadows, our data indicate that between 41 and 66 gC m−2 yr−1 originates from seagrass production. Using our global average for allochthonous carbon trapped in seagrass sediments together with a recent estimate of global average net community production, we estimate that carbon burial in seagrass meadows is between 48 and 112 Tg yr−1, showing that seagrass meadows are natural hot spots for carbon sequestration

Storage Time of Transfused Red Blood Cell and the Risk of Alloimmunization

Clinical epidemiolog