[1] Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15 N/ 14 N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle.Citation: Robinson, R. S., et al. (2012), A review of nitrogen isotopic alteration in marine sediments, Paleoceanography, 27, PA4203,

Abstract. We investigated the spatial distribution and seasonal variation of dissolved inorganic nitrogen in a large perturbed estuary, the Pearl River Estuary, based on three cruises conducted in winter (January 2005), summer (August 2005) and spring (March 2006). On-site incubation was also carried out for determining ammonium and nitrite oxidation rates (nitrification rates). We observed a year-round pattern of dramatic decrease in NH , accompanied by extremely high concentrations of ammonia (up to >800 µmol L −1 ) and nitrate (up to >300 µmol L −1 ). In summer, the upper estuary showed higher nitrification rates (ammonia oxidation rate ∼1.5-33.1 µmol N L −1 d −1 , nitrite oxidation rate ∼0.6-32.0 µmol N L −1 d −1 ) with lower concentrations of ammonia (<350 µmol L −1 ) and nitrate (<120 µmol L −1 ). The Most Probable Number test showed relatively lower nitrifier abundance in summer at most sampling stations, indicating a greater specific nitrification rate per cell in the warm season. Temperature appeared to control nitrification rates to a large degree in different seasons. Spatial variability of nitrification rates appeared to be controlled by a combination of many other factors such as nutrient concentrations, nitrifier abundance and dissolved oxygen (DO) concentrations. In addition to aerobic respiration, nitrification contributed significantly to the consumption of DO and production of free CO 2 at upper estuary. Nitrification-induced consumption accounted for up to approximately one third of the total waCorrespondence to: M. Dai (mdai@xmu.edu.cn) ter column community DO consumption in the upper estuary during the surveyed periods, boosting environmental stress on this large estuarine ecosystem.

We examined the dynamics of dissolved inorganic nitrogen (DIN, nitrate + nitrite), dissolved inorganic phosphorus (DIP), and silicate (Si(OH) 4 ) in the northern shelf of the South China Sea in summer, which is under a complex hydrodynamic scheme largely shaped by river plume and coastal upwelling, along with the enhanced biological consumption of nutrients therein. The Pearl River plume, with high nutrient concentrations (, 0.1-14.2 mmol L 21 for DIN, , 0.02-0.10 mmol L 21 for DIP, and , 0.2-18.9 mmol L 21 for Si(OH) 4 ), occupied a large area of the middle shelf (salinity , 33.5). The nearshore area had high nutrient concentrations apparently sourced from subsurface nutrient-replete waters through wind-driven coastal upwelling. These nutrient levels were significantly elevated relative to those on the oligotrophic outer shelf where DIN, DIP, and Si(OH) 4 concentrations dropped to , 0.1 mmol L 21 , , 0.02-0.03 mmol L 21, and , 2.0 mmol L 21, respectively. A three end-member mixing model was constructed based on potential temperature and salinity conservation to assess biological consumption of inorganic nutrients, which was denoted by D and defined by the deviation from conservative mixing. In the coastal upwelling zone and deep chlorophyll maximum layer, the nutrient uptake ratio DDIN : DDIP was 16.7, which is the classic Redfield ratio. In contrast, in the river plume the uptake ratio was 61.3 6 8.7. We believed that an alternative non-DIP source likely contributed to this higher DIN : DIP consumption ratio in the river plume regime. Meanwhile, Si(OH) 4 showed predominant consumption in the river plume and a combination of regeneration and consumption along the path of the coastal upwelling current.

Small mountainous rivers have received increasing attention in recent years because of their high yield of sediments and particulate organic matter. A one-year (1993-l 994) study of the fluxes of the total suspended matter (TSM) and particulate organic carbon (POC) in different parts of Lanyang Hsi, a typical small mountainous river in subtropical northeastern Taiwan indicated that the mean TSM and POC yields of the whole drainage area were high (3,600 and 23 g m-2 yr-I, respectively) but that the yields of less disturbed (control) tributaries were only l&, of the mean yields of the whole area. Radiocarbon analysis by accelerator mass spectrometry gave very old apparent ages (> 10,000 yr) to the POC in the main channel, indicating that > 70% of the POC produced in the drainage basin was probably derived from kcrogen in the bedrock. TSM yields for 1970-1991, calculated from historical records of runoffs and TSM concentrations, showed large fluctuations from year to year; howcvcr, the average yield after 1976 (8,335 g m-2 yr