Tim Rixen

Seasonality and interannual variability of particle-fluxes to the deep Arabian Sea

Abstract Long-term sediment trap studies have been carried out since 1986 at three locations in the western, central and eastern Arabian Sea. Here we present total and bulk component fluxes measured for 3 years at the central station and for 4 years at the western and eastern stations. Particulate fluxes to the deep sea are controlled by the monsoons with generally higher fluxes during the SW and NE monsoons and lower fluxes during the intermonsoon periods. The increase of particle fluxes occurs simultaneously with a drop in surface water temperature, induced by wind-or convective-mixing and an associated entrainment of nutrients into the euphotic zone. More than 50% of the annual particle fluxes to the deep sea occurs during the SW monsoon at the western location due to the prolonged influence of the monsoonal upwelling as indicated by increased biogenic carbonate and opal fluxes. However, the opal fluxes peak a month later than the carbonate fluxes. The delayed onset of opal flux peak appears to be controlled by the observed premonsoon silica distribution in the Arabian Sea, where the subsurface waters are silica depleted down to the thermocline at 150 m. At the central location particle fluxes are of similar magnitude during the SW and NE monsoons. The interannual variability of particle fluxes at the eastern location is determined by the NE monsoon. At the western and central locations, on the other hand, maximum interannual variability of fluxes occurs during the SW monsoon and particle fluxes were higher during years of stronger SW monsoon. The results further suggest that, apart from monsoon strength, geographic shifts of the area of maximum wind-stress may produce significant variabilities in particle fluxes to the deep ocean at the western Arabian Sea site.

A revised nitrogen budget for the Arabian Sea

Despite its importance for the global oceanic nitrogen (N) cycle, considerable uncertainties exist about the N fluxes of the Arabian Sea. On the basis of our recent measurements during the German Arabian Sea Process Study as part of the Joint Global Ocean Flux Study (JGOFS) in 1995 and 1997, we present estimates of various N sources and sinks such as atmospheric dry and wet depositions of N aerosols, pelagic denitrification, nitrous oxide (N2O) emissions, and advective N input from the south. Additionally, we estimated the N burial in the deep sea and the sedimentary shelf denitrification. On the basis of our measurements and literature data, the N budget for the Arabian Sea was reassessed. It is dominated by the N loss due to denitrification, which is balanced by the advective input of N from the south. The role of N fixation in the Arabian Sea is still difficult to assess owing to the small database available; however, there are hints that it might be more important than previously thought. Atmospheric N depositions are important on a regional scale during the intermonsoon in the central Arabian Sea; however, they play only a minor role for the overall N cycling. Emissions of N2O and ammonia, deep-sea N burial, and N inputs by rivers and marginal seas (i.e., Persian Gulf and Red Sea) are of minor importance. We found that the magnitude of the sedimentary denitrification at the shelf might be ∼17% of the total denitrification in the Arabian Sea, indicating that the shelf sediments might be of considerably greater importance for the N cycling in the Arabian Sea than previously thought. Sedimentary and pelagic denitrification together demand ∼6% of the estimated particulate organic nitrogen export flux from the photic zone. The main northward transport of N into the Arabian Sea occurs in the intermediate layers, indicating that the N cycle of the Arabian Sea might be sensitive to variations of the intermediate water circulation of the Indian Ocean.

Coupling between SW monsoon-related surface and deep ocean processes as discerned from continuous particle flux measurements and correlated satellite data

Particle flux data obtained by time series sediment traps deployed at water depths of approximately 3000 m in the western, central, and eastern Arabian Sea since 1986 were compared with wind speeds derived from measurements made by microwave radiometer flying on polar orbiting satellites and sea surface temperatures (SSTs) provided by the Physical Oceanography Distributed Active Archive Center at Jet Propulsion Laboratory. This comparison has allowed us to trace the link between the oceanographic and biological processes related to the development of the SW monsoon with the pattern and interannual variability of particle fluxes to the interior of the Arabian Sea. We could recognize the well-known upwelling systems along the coasts of Somalia and Oman as well as open ocean upwelling at the beginning of the SW monsoon. Both open ocean upwelling and coastal upwelling off Oman cause a cooling of surface waters at our western and central Arabian Sea stations. When SSTs fall below their long-term average, an increase in fluxes which are dominated by coccolithophorid-derived carbonates occurs. The timing of this increase is determined by the rate of surface water cooling. Further intensification of upwelling as the SW monsoon progresses causes additional increases in biogenic opal fluxes denoting diatom blooms in the overlying waters. The total fluxes during this period are the highest measured in the open Arabian Sea. At the central Arabian Sea location the fluxes are only randomly affected by these blooms. The particle flux in the eastern Arabian Sea is as high as in the central Arabian Sea but is influenced by a weaker upwelling system along the Indian coast. The observed interannual variability in the pattern of particle fluxes during the SW monsoons is most pronounced in the western Arabian Sea. This is controlled by the intensity of the upwelling systems on the one hand and the transport of cold, nutrient-poor, south equatorial water into the Oman region on the other. The latter effect, which is strongest during the SW monsoon with highest recorded wind speeds, reduces the influence of upwelling and the related particle fluxes at the western Arabian Sea station, where highest fluxes occur during SW monsoons with moderate wind speeds. Thus coastal and open ocean upwelling are most effective in transferring biogenic matter to the deep sea during the SW monsoons of intermediate strength.

Sedimentation in the western Arabian Sea the role of coastal and open-ocean upwelling

Monsoon-induced coastal and open-ocean upwelling explain 84% of the variations of the organic carbon fluxes measured in the deep western Arabian Sea. In this paper, sea-level measurements, satellite-derived wind speeds, sea surface temperatures, and nutrient profiles are used to discern the relative importance of these factors on fluxes measured during nine years of continuous sediment trap deployments. This exercise shows: (i) the increase in fluxes observed during the initial stages of the SW monsoons are caused by open-ocean upwelling, which develops faster than the coastal upwelling; (ii) coastal upwelling triggers diatom blooms from nutrients from subsurface water and sediment resuspension and, more importantly, by injecting resting stages of diatoms back into the euphotic zone; (iii) silica depletion resulting from diatom blooms in laterally advecting water masses leads to a replacement of diatoms by other nitrate-limited organisms; (iv) organic carbon fluxes to the deep Arabian Sea increase in response to an intensification of both coastal and open-ocean upwelling; weak coastal upwelling and strong open-ocean upwelling also increase organic carbon fluxes. The varying dominance of their influence is reflected in the timing and the composition of the peak fluxes; (v) the link between organic carbon flux and monsoon strength is non-linear probably due to changes in the surface currents and to vigorous turbulence in the surface water during strong SW monsoons. These processes could reduce the organic carbon flux in the western Arabian Sea by about 65%.

The influence of the SW monsoon on the deep-sea organic carbon cycle in the Holocene

Abstract Results from long-term sediment trap experiments carried out since 1986 in the western, central, and eastern Arabian Sea are combined with satellite-derived wind fields and paleoceanographic information to link the intensity of the SW monsoon to organic carbon fluxes and its preservation in sediments. The SW monsoon is characterized by the low-level jet (Findlater Jet) that crosses the Arabian Sea almost parallel to the Arabian coast. The intensity of the Findlater Jet mainly controls the velocities of upwelling that occurs to the northeast of the jet. Since up welling, in turn, mainly governs the organic carbon fluxes in the western Arabian Sea, variation in the strength of the Findlater Jet is the dominant factor determining the organic carbon fluxes on seasonal time scales. Changes in the subsurface nutrient concentrations due to variations in the surface ocean current systems seem to be another factor influencing the organic carbon fluxes, mostly on interannual time scales. The translation of sedimentary organic carbon burial rates into organic carbon fluxes according to Jahnke (1996). Global Biogeochemical Cycles 10, 71–88) allows us to extend our reflections also to a millennium time scale. This indicates that changes in the SW monsoon intensity as observed during the last decade could almost account for the range of organic carbon fluxes deciphered from the Holocene record.

Dissolved oxygen and its response to eutrophication in a tropical black water river

The Siak is a typical, nutrient-poor, well-mixed, black water river in central Sumatra, Indonesia, which owes its brown color to dissolved organic matter (DOM) leached from surrounding, heavily disturbed peat soils. We measured dissolved organic carbon (DOC) and oxygen concentrations along the river, carried out a 36-h experiment in the province capital Pekanbaru and quantified organic matter and nutrient inputs from urban wastewater channels into the Siak. In order to consider the complex dynamic of oxygen in rivers, a box-diffusion model was used to interpret the measured data. The results suggest that the decomposition of soil derived DOM was the main factor influencing the oxygen concentration in the Siak which varied between approximately 100 and 140 micromol l(-1). Additional DOM input caused by wastewater discharges appeared to reduce the oxygen concentrations by approximately 20 micromol l(-1) during the peak-time in household water use in the early morning and in the early evening. Associated enhanced nutrient inputs appear to reduce the impact of the anthropogenic DOM by favoring the photosynthetic production of oxygen in the morning. A reduction of 20 micromol l(-1), which although perhaps not of great significance in Pekanbaru, has strong implications for wastewater management in the fast developing areas downstream Pekanbaru where oxygen concentrations rarely exceed 20 micromol l(-1).

The impact of disturbed peatlands on river outgassing in Southeast Asia

River outgassing has proven to be an integral part of the carbon cycle. In Southeast Asia, river outgassing quantities are uncertain due to lack of measured data. Here we investigate six rivers in Indonesia and Malaysia, during five expeditions. CO2 fluxes from Southeast Asian rivers amount to 66.9±15.7 Tg C per year, of which Indonesia releases 53.9±12.4 Tg C per year. Malaysian rivers emit 6.2±1.6 Tg C per year. These moderate values show that Southeast Asia is not the river outgassing hotspot as would be expected from the carbon-enriched peat soils. This is due to the relatively short residence time of dissolved organic carbon (DOC) in the river, as the peatlands, being the primary source of DOC, are located near the coast. Limitation of bacterial production, due to low pH, oxygen depletion or the refractory nature of DOC, potentially also contributes to moderate CO2 fluxes as this decelerates decomposition.

Monsoonal and ENSO Impacts on Particle Fluxes and the Biological Pump in the Indian Ocean

Data obtained by sediment trap experiments in the Indian Ocean were evaluated in conjunction with additional information derived mainly from the Joint Global Ocean Flux Study (JGOFS) expeditions in 1994/1995. Our results indicate that wind-driven upwelling, thermohaline mixing, and freshwater inputs are the main physical processes through which the monsoon drives the biogeochemical fluxes in the Indian Ocean. The upwelling system in the Arabian Sea seems to be bottom-up controlled and very sensitive to iron supply affecting system immanent threshold concentrations and to nitrate reduction rates in the mid-water oxygen minimum zones (OMZs). On geologic timescales, nitrate reduction rates were strongly linked to Northern Hemispheric climate variations, whereby the thermohaline mixing and the associated mid-water formation in the northern Arabian Sea could have played a key role. The ratios between organic and inorganic carbon of particles exported from the surface ocean indicate that the biologically mediated CO 2 uptake, referred to as the biological pump, is low in the Arabian Sea during the high productive upwelling period. The biological pump seems to be strongest along the freshwater-influenced continental margins in the eastern Indian Ocean where the climate anomaly El Nino―Southern Oscillation (ENSO) influences the biological pump by its impact on the precipitation rates and the river discharges. Associated physical processes such as the capping effect amplify the ENSO impact on the CO 2 degassing, whereby ENSO increases/decreases the CO 2 degassing into the atmosphere in its negative (El Nino) and positive mode (La Nina).

Late Holocene primary productivity and sea surface temperature variations in the northeastern Arabian Sea: Implications for winter monsoon variability

Variability in the oceanic environment of the Arabian Sea region is strongly 34 influenced by the seasonal monsoon cycle of alternating wind directions. Prominent and well studied is the summer monsoon, but much less is known about late Holocene changes in winter monsoon strength with winds from the northeast that drive convective mixing and highsurface ocean productivity in the northeastern Arabian Sea. To establish the first high resolution record of winter monsoon variability for the late Holocene, we analyzed alkenone derived sea surface temperature (SST) variations and proxies of primary productivity (organic carbon and δ15N) in a well-laminated sediment core from the Pakistan continental margin. Increased summer monsoon and weak winter monsoon intensities off Pakistan are indicated from 400 B.C. to 700 A.D. by reduced productivity and relatively high SST. At about 700 A.D. the intensity of the winter monsoon increased off Pakistan as indicated by a trend to lower SST. We infer that winter monsoon was still weak from 700 to 1400 A.D., because primary production did not increase despite decreasing SST. Declining SST and elevated biological production from 1400 to 1900 A.D. suggest invigorated convective winter mixing by strengthening winter monsoon circulation, most likely a regional expression of colder climate conditions during the Little Ice Age on the Northern Hemisphere. The comparison of winter monsoon intensity with records of summer monsoon intensity suggests that an inverse relationship between summer and winter monsoon strength exists in the Asian monsoon system during the late Holocene, effected by shifts in the Intertropical Convergence Zone.

Dynamics in benthic community composition and influencing factors in an upwelling-exposed coral reef on the Pacific coast of Costa Rica

Seasonal upwelling at the northern Pacific coast of Costa Rica offers the opportunity to investigate the effects of pronounced changes in key water parameters on fine-scale dynamics of local coral reef communities. This study monitored benthic community composition at Matapalo reef (10.539°N, 85.766°W) by weekly observations of permanent benthic quadrats from April 2013 to April 2014. Monitoring was accompanied by surveys of herbivore abundance and biomass and measurements of water temperature and inorganic nutrient concentrations. Findings revealed that the reef-building corals Pocillopora spp. exhibited an exceptional rapid increase from 22 to 51% relative benthic cover. By contrast, turf algae cover decreased from 63 to 24%, resulting in a corresponding increase in crustose coralline algae cover. The macroalga Caulerpa sertularioides covered up to 15% of the reef in April 2013, disappeared after synchronized gamete release in May, and subsequently exhibited slow regrowth. Parallel monitoring of influencing factors suggest that C. sertularioides cover was mainly regulated by their reproductive cycle, while that of turf algae was likely controlled by high abundances of herbivores. Upwelling events in February and March 2014 decreased mean daily seawater temperatures by up to 7 °C and increased nutrient concentrations up to 5- (phosphate) and 16-fold (nitrate) compared to mean values during the rest of the year. Changes in benthic community composition did not appear to correspond to the strong environmental changes, but rather shifted from turf algae to hard coral dominance over the entire year of observation. The exceptional high dynamic over the annual observation period encourages further research on the adaptation potential of coral reefs to environmental variability.