Burdanowitz, N., Gaye, B., Hilbig, L., Lahajnar, N., Lückge, A., Rixen, T., & Emeis, K.-C. (2019): Holocene monsoon and sea level-related changes of sedimentation in the northeastern Arabian Sea. Deep Sea Research Part II: Topical Studies in Oceanography, ISSN 0967-0645, doi:10.1016/j.dsr2.2019.03.003
The Indian Monsoon and the westerlies strongly influence the sedimentation in the northeastern Arabian Sea by impacting rainfall and erosion on land and on biogeochemical processes in the ocean. To disentangle the terrestrial and oceanic processes, we analysed mineralogical and bulk geochemical components of a Holocene sediment core offshore Pakistan. Endmember modelling of grain sizes and principal component analyses (PCA) of major and trace elements identify the origin of sediments and their dominant mode of transport. Sedimentation processes during the early Holocene (10.8–8.2 ka BP) were influenced by the post-glacial sea level rise and orbitally forced strengthening of the Indian summer monsoon (ISM) and westerlies. This led to a shift from rather terrestrial-dominated towards a marine-dominated sedimentation, whereas the fluvial source shifted from the Makran rivers to the Hab River near Karachi. During the mid-Holocene (8.2–4.2 ka BP) a combination of weakening ISM and southward displacement of the ITCZ enhanced the influence of the westerlies, together decreasing river discharges and enhancing aeolian input (probably from the Sistan Basin region). This trend continued during the last ca. 4 ka when the increasing aridification of the Hab River catchment further increased the aeolian inputs. Solar and lunar driven short-term variations as well as Bond events known from the North Atlantic Ocean superpose these trends. They lead to a pronounced increase of fluvial inputs between 8.6–8.4 ka BP and at ca. 3 ka BP as well as to dry events around 4.2 ka and 1.2–1 ka BP. Our study highlights the increasing influence of the westerlies on the sedimentation processes in the northeastern Arabian Sea towards the late Holocene.
Rixen, T., Gaye, B., & Emeis, K.-C. (2019): The monsoon, carbon fluxes, and the organic carbon pump in the northern Indian Ocean. Progress in Oceanography, Volume 175, Pages 24-39, doi:10.1016/j.pocean.2019.03.001
Time series sediment trap experiments were carried out at fifteen sites in the northern Indian Ocean between 1986 and 2007. The data on particle flux rates and composition are analyzed in combination with satellite-derived estimates of primary production and results of surface ocean studies during the Joint Global Ocean Flux Study in the Arabian Sea (JGOFS-Indik). The data highlight the influence of the monsoon on the transport of organic carbon into the deep sea and the associated functioning of the organic carbon pump.
The results illustrate the well-known concept of export production, which is driven by inputs of nutrients from the aphotic zone and external reservoirs (the atmosphere and the land) into the euphotic zone. The monsoon drives the organic carbon export through its impact on the physical nutrient supply mechanisms, such as upwelling, vertical mixing, and river discharges. Eolian dust and especially riverine supply of lithogenic matter increase organic carbon fluxes by accelerating the transport of organic matter into the deep sea. Nevertheless, it is preferentially respired in the sub-thermocline and the resulting trapping of remineralized nutrients at this water-depth enforces the influence of upwelling and vertical mixing on the organic carbon fluxes which in the northern Indian Ocean are among the highest worldwide.
Model experiments and measured organic carbon burial rates indicate that a weakening of the summer monsoon strength hardly affected the long-term annual average organic carbon export flux into the deep sea during the last approximately 7000 years. In addition to the summer and winter monsoon strength, which are assumed to be inversely related to each other, monsoon-driven physical impacts on the nutrient trapping efficiency seem to have kept organic carbon fluxes at a high level. A feedback mechanism caused by negative impacts of oxygen concentrations on the respiration and thus nutrient trapping efficiency apparently prevents the development of anoxia to the point where sulfate reduction occurs and sets an upper limit to organic carbon fluxes. Whether changes in the phytoplankton community structure observed in recent decades indicate that this self-regulating system is becoming unstable is open to question.
Rixen, T., Gaye, B., Emeis, K.-C., & Ramaswamy, V. (2019): The ballast effect of lithogenic matter and its influences on the carbon fluxes in the Indian Ocean. Biogeosciences, 16, pp 485-503, doi:10.5194/bg-16-485-2019
Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with satellite observations illustrate the influence of primary production and the ballast effect on organic carbon flux into the deep sea. They suggest that primary production is the main control on the spatial variability of organic carbon fluxes at most of our study sites in the Indian Ocean, except at sites influenced by river discharges. At these sites the spatial variability of organic carbon flux is influenced by lithogenic matter content. To quantify the impact of lithogenic matter on the organic carbon flux, the densities of the main ballast minerals, their flux rates and seawater properties were used to calculate sinking speeds of material intercepted by sediment traps. Sinking speeds in combination with satellite-derived export production rates allowed us to compute organic carbon fluxes. Flux calculations imply that lithogenic matter ballast increases organic carbon fluxes at all sampling sites in the Indian Ocean by enhancing sinking speeds and reducing the time of organic matter respiration in the water column. We calculated that lithogenic matter content in aggregates and pellets enhances organic carbon flux rates on average by 45 % and by up to 62 % at trap locations in the river-influenced regions of the Indian Ocean. Such a strong lithogenic matter ballast effect explains the fact that organic carbon fluxes are higher in the low-productive southern Java Sea compared to the high-productive western Arabian Sea. It also implies that land use changes and the associated enhanced transport of lithogenic matter from land into the ocean may significantly affect the CO2 uptake of the organic carbon pump in the receiving ocean areas.