Description: 13C tracer experiments were conducted at sites spanning the steep oxygen, organic matter, and biological community gradients across the Arabian Sea oxygen minimum zone, in order to quantify the role that benthic fauna play in the short-term processing of organic matter (OM) and to determine how this varies among different environments. Metazoan macrofauna and macrofauna-sized foraminiferans took up as much as 56 +/- 13 mg of added C m**-2 (685 mg C m**-2 added) over 25 d, and at some sites this uptake was similar in magnitude to bacterial uptake and/or total respiration. Bottom-water dissolved oxygen concentrations exerted a strong control over metazoan macrofaunal OM processing. At oxygen concentrations 〉7 µmol/L (0.16 ml/L), metazoan macrofauna were able to take advantage of abundant OM and to dominate OM uptake, while OM processing at O2 concentrations of 5.0 µmol/L (0.11 ml/L) was dominated instead by (macrofaunal) foraminiferans. This led us to propose the hypothesis that oxygen controls the relative dominance of metazoan macrofauna and foraminifera in a threshold manner, with the threshold lying between 5 and 7 µmol/L (0.11 to 0.16 ml/L). Large metazoan macrofaunal biomass and high natural concentrations of OM were also associated with rapid processing of fresh OM by the benthic community. Where they were present, the polychaete Linopherus sp. and the calcareous foraminiferan Uvigerina ex gr. semiornata, dominated the uptake of OM above and below, respectively, the proposed threshold concentrations of bottom-water oxygen.

Description: The Pakistan Margin is characterised by a strong mid-water oxygen minimum zone (OMZ) that intercepts the seabed at bathyal depths (1501300 m). We investigated whether faunal abundance and diversity trends were similar among protists (foraminiferans and gromiids), metazoan macrofauna and megafauna along a transect (1401850 m water depth) across the OMZ during the 2003 intermonsoon (MarchMay) and late/post-monsoon (AugustOctober) seasons. All groups exhibited some drop in abundance in the OMZ core (250500 m water depth; O2: 0.100.13 mL/L=4.465.80 μM) but to differing degrees. Densities of foraminiferans 〉63 μm were slightly depressed at 300 m, peaked at 738 m, and were much lower at deeper stations. Foraminiferans 〉300 μm were the overwhelmingly dominant macrofaunal organisms in the OMZ core. Macrofaunal metazoans reached maximum densities at 140 m depth, with additional peaks at 850, 940 and 1850 m where foraminiferans were less abundant. The polychaete Linopherus sp. was responsible for a macrofaunal biomass peak at 950 m. Apart from large swimming animals (fish and natant decapods), metazoan megafauna were absent between 300 and 900 m (O2 〈0.140.15 mL/L=6.256.69 μM) but were represented by a huge, ophiuroid-dominated abundance peak at 1000 m (O2 0.150.18 mL/L=6.698.03 μM). Gromiid protists were confined largely to depths below 1150 m (O2 〉0.2 mL/L=8.92 μM). The progressively deeper abundance peaks for foraminiferans (〉63 μm), Linopherus sp. and ophiuroids probably represent lower OMZ boundary edge effects and suggest a link between body size and tolerance of hypoxia. Macro- and megafaunal organisms collected between 800 and 1100 m were dominated by a succession of different taxa, indicating that the lower part of the OMZ is also a region of rapid faunal change. Species diversity was depressed in all groups in the OMZ core, but this was much more pronounced for macrofauna and megafauna than for foraminiferans. Oxygen levels strongly influenced the taxonomic composition of all faunal groups. Calcareous foraminiferans dominated the seasonally and permanently hypoxic sites (136300 m); agglutinated foraminiferans were relatively more abundant at deeper stations where oxygen concentrations were 〉0.13 mL/L(=5.80 μM). Polychaetes were the main macrofaunal taxon within the OMZ; calcareous macrofauna and megafauna (molluscs and echinoderms) were rare or absent where oxygen levels were lowest. The rarity of larger animals between 300 and 700 m on the Pakistan Margin, compared with the abundant macrofauna in the OMZ core off Oman, is the most notable contrast between the two sides of the Arabian Sea. This difference probably reflects the slightly higher oxygen levels and better food quality on the western side.

Description: The balance between degradation and preservation of sedimentary organic carbon (OC) is important for global carbon and oxygen cycles1. The relative importance of different mechanisms and environmental conditions contributing to marine sedimentary OC preservation, however, remains unclear2,3,4,5,6,7,8. Simple organic molecules can be geopolymerized into recalcitrant forms by means of the Maillard reaction5, although reaction kinetics at marine sedimentary temperatures are thought to be slow9,10. More recent work in terrestrial systems suggests that the reaction can be catalysed by manganese minerals11,12,13, but the potential for the promotion of geopolymerized OC formation at marine sedimentary temperatures is uncertain. Here we present incubation experiments and find that iron and manganese ions and minerals abiotically catalyse the Maillard reaction by up to two orders of magnitude at temperatures relevant to continental margins where most preservation occurs4. Furthermore, the chemical signature of the reaction products closely resembles dissolved and total OC found in continental margin sediments globally. With the aid of a pore-water model14, we estimate that iron- and manganese-catalysed transformation of simple organic molecules into complex macromolecules might generate on the order of approximately 4.1 Tg C yr−1 for preservation in marine sediments. In the context of perhaps only about 63 Tg C yr−1 variation in sedimentary organic preservation over the past 300 million years6, we propose that variable iron and manganese inputs to the ocean could exert a substantial but hitherto unexplored impact on global OC preservation over geological time.