Miocene phosphorites from the Murray Ridge, Northwestern Arabian Sea

Phosphorites from the Murray Ridge, NW Arabian Sea comprise nodules, bioclasts, and bone fragments. The nodules are made up of a homogeneous, light-colored phosphate nucleus consisting of Rivulariacean filamentous cyanobacteria and a thin dark-grey colored phosphate cortex showing abundant microbial filaments and microborings. The bioclasts comprise of ∼14-14.5 Ma old planktonic foraminifers, accepted as the time of deposition. Spherical to ovoid-shaped apatite microparticles resembling fossil bacteria are distinct components in the bioclasts. Bone fragments exhibit apatite fillings. The nodules and bone fragments consist entirely of carbonate fluorapatite (CFA) with low Al, K, and Th concentrations suggesting absence of continental detritus. Shale-normalized REE patterns of the samples support a seawater-derived composition. The highly uniform initial εNd values of -4.8 to -5.1 are interpreted as the seawater value at the onset of phosphatization ∼14 Ma ago. In contrast, 87Sr/86Sr ratios show a large range of 0.709055 to 0.709124 corresponding to unusually young stratigraphic ages of ∼1 to 3 Ma. The data are interpreted as evidence for post-depositional Sr exchange of the recrystallizing phosphorites with fluids isotopically not much different from modern seawater. It is concluded that the phosphorites formed under oxic, shallow-water conditions where microbial populations assimilated phosphorus primarily from seawater and mediated precipitation of CFA during early diagenesis at the sediment-water interface on different substrates

Remarkable structural resistance of a nanoflagellatedominated plankton community to iron fertilization during the Southern Ocean experiment LOHAFEX

The genesis of phytoplankton blooms and the fate of their biomass in iron-limited, high-nutrient−low-chlorophyll regions can be studied under natural conditions with ocean iron fertilization (OIF) experiments. The Indo-German OIF experiment LOHAFEX was carried out over 40 d in late summer 2009 within the cold core of a mesoscale eddy in the productive southwest Atlantic sector of the Southern Ocean. Silicate concentrations were very low, and phytoplankton biomass was dominated by autotrophic nanoflagellates (ANF) in the size range 3−10 μm. As in all previous OIF experiments, the phytoplankton responded to iron fertilization by increasing the maximum quantum yield (Fv/Fm) and cellular chlorophyll levels. Within 3 wk, chlorophyll levels tripled and ANF biomass doubled. With the exception of some diatoms and dinoflagellates, the biomass levels of all other groups of the phyto- and protozooplankton (heterotrophic nanoflagellates, dinoflagellates and ciliates) remained remarkably stable throughout the experiment both inside and outside the fertilized patch. We attribute the unusually high biomass attained and maintained by ANF to the absence of their grazers, the salps, and to constraints on protozooplankton grazers by heavy predation exerted by the large copepod stock. The resistance to change of the ecosystem structure over 38 d after fertilization, indicated by homogeneity at regional and temporal scales, suggests that it was locked into a stable, mature state that had evolved in the course of the seasonal cycle. The LOHAFEX bloom provides a case study of a resistant/robust dynamic equilibrium between auto- and heterotrophic ecosystem components resulting in low vertical flux both inside and outside the patch despite high biomass levels

The Ocean is Losing its Breath: Declining Oxygen in the Worlds Ocean and Coastal Waters

'The Ocean is Losing its Breath' presents a summary of scientific experiments, observations and numerical models addressing the following questions: How has the oxygen content in the open ocean and coastal waters changed over the past century and through geological time? What are the mechanisms behind this oxygen decline? How is ocean oxygen content predicted to change over the rest of the twenty-first century? What are the consequences of low and declining oxygen concentrations in the marine environment? This document was prepared by a group of concerned scientists from across the world, the IOC expert group, the Global Ocean Oxygen Network GO2 NE, established in 2016, which is committed to providing a global and multidisciplinary view on deoxygenation, with a focus on understanding its various aspects and impacts

Multidisciplinary Observing in the World Ocean’s Oxygen Minimum Zone Regions: From Climate to Fish — The VOICE Initiative

Multidisciplinary ocean observing activities provide critical ocean information to satisfy ever-changing socioeconomic needs and require coordinated implementation. The upper oxycline (transition between high and low oxygen waters) is fundamentally important for the ecosystem structure and can be a useful proxy for multiple observing objectives connected to eastern boundary systems (EBSs) that neighbor oxygen minimum zones (OMZs). The variability of the oxycline and its impact on the ecosystem (VOICE) initiative demonstrates how societal benefits drive the need for integration and optimization of biological, biogeochemical, and physical components of regional ocean observing related to EBS. In liaison with the Global Ocean Oxygen Network, VOICE creates a roadmap toward observation-model syntheses for a comprehensive understanding of selected oxycline-dependent objectives. Local to global effects, such as habitat compression or deoxygenation trends, prompt for comprehensive observing of the oxycline on various space and time scales, and for an increased awareness of its impact on ecosystem services. Building on the Framework for Ocean Observing (FOO), we present a first readiness level assessment for ocean observing of the oxycline in EBS. This was to determine current ocean observing design and future needs in EBS regions (e.g., the California Current System, the Equatorial Eastern Pacific off Ecuador, the Peru–Chile Current system, the Northern Benguela off Namibia, etc.) building on the FOO strategy. We choose regional champions to assess the ocean observing design elements proposed in the FOO, namely, requirement processes, coordination of observational elements, and data management and information products and the related best practices. The readiness level for the FOO elements was derived for each EBS through a similar and very general ad hoc questionnaire. Despite some weaknesses in the questionnaire design and its completion, an assessment was achievable. We found that fisheries and ecosystem management are a societal requirement for all regions, but maturity levels of observational elements and data management and information products differ substantially. Identification of relevant stakeholders, developing strategies for readiness level improvements, and building and sustaining infrastructure capacity to implement these strategies are fundamental milestones for the VOICE initiative over the next 2–5 years and beyond

Reduced iron associated with secondary nitrite maxima in the Arabian Sea

Dissolved iron and Fe(II) were measured in the oxygen minimum zone (OMZ) of the Arabian Sea in September 2004. The OMZ is a well-demarcated feature characterized by high rates of denitrification, and a deep nitrite maximum coinciding with oxygen levels below 1 μmol L-1. This zone is significantly enriched in dissolved Fe relative to overlying and underlying waters and up to 50% of the dissolved Fe is present as Fe(II). The maxima in Fe(II) are at the same depth as the deep nitrite maxima, centered around 200-250 m. They coincide with a local maximum in total dissolved Fe, suggesting that Fe accumulates at this depth because of the greater solubility of Fe(II) over Fe(III). Fe(II) is thermodynamically unstable even at submicromolar oxygen levels, so active biological reduction is the most plausible source. To our knowledge, this is the first report of a potential link between Fe reduction, elevated dissolved Fe concentrations, and nitrite accumulation within an OMZ. Denitrification has a high Fe requirement associated with the metalloenzymes for nitrate and nitrite reduction, so in situ redox cycling of Fe has important implications for the nitrogen cycle

Denitrification in the Arabian Sea

Estimates of the amount of denitrification in the Arabian Sea are inconsistent, and so two methods of calculation of 'original' nitrate concentrations used in earlier studies have been reviewed. Nitrite at intermediate depths was not restricted to the Persian Gulf water; concentrations were maximal much shallower than the depths of the Persian Gulf water. The nitrate-salinity relationship used to calculate 'original' nitrate concentrations in a recent study would lead to an underestimate of nitrate deficits because the approach requires denitrification to be restricted to Persian Gulf water. A method involving oxidative ratios has been modified and used to calculate nitrate deficits. The deficits so calculated lie between the values reported earlier. The degree of denitrification decreases steadily southward. The results indicate that denitrification extends up to 11 or 12°N latitude. Taking 30 years as the average residence time of water for the layer in which denitrification occurs, the denitrification rate in the Arabian Sea has been calculated as about 3.2 × 1012 gy-1. This represents about 5% of the annual global denitrification

(Table 1) Age determination of sediment core AAS-62/2

Variations in the Indian summer monsoon (ISM) intensity during the last 16.7 ka have been studied using organic carbon (Corg), d15N of sedimentary organic matter, CaCO3, sediment texture, d18OC, and Mg/Ca-derived sea surface temperature, d18O of sea water and sea surface salinity, in a 14C-dated sediment core from the eastern Arabian Sea. The d18O in water and planktonic foraminifera shells off the central west coast of India may be controlled by the ISM intensity as this area receives high precipitation and land runoff. Also, the Corg and CaCO3 contents of sediments and d15N of sedimentary organic matter may be linked to ISM-induced productivity and denitrification. The results of the present study reveal that between 16 and 15.2 ka BP, the ISM was weak with minor fluctuations and started intensifying around 15.2 ka BP, at the onset of the Bølling-Ållerød (B-A) event. The B-A event is characterized by higher water column denitrification rates comparable to the present day. The ISM signatures observed in the d18OC record of B-A event compare well with those from Timta cave of the western Himalayas and also the Asian summer monsoon signatures from the Hulu caves in China and warming signatures in Greenland Ice Sheet Project 2 (GISP2) suggesting atmospheric teleconnections through Intertropical Convergence Zone. The boundary between the Younger Dryas and the Holocene is discernible with small episodes of abrupt events of increased ISM intensity. This decrease in d18OC values at ~11.8 ka BP is contemporary with June solar insolation maximum at 30° north and the increase in methane in the GISP2 ice core supporting episodes of warmer climate and increase in ISM intensity. The ISM seems to have been most stable between 7 and 5.6 ka BP. The core exhibits periodicity of 500 years that is comparable to the Atlantic water formation and the Chinese monsoon

Phytoplankton dynamics in a seasonal stratified reservoir (Tillari), Western India

Phytoplankton are the primary producers in all the aquatic ecosystems and play an important role in key biogeochemical processes that are linked to the higher trophic levels and climate variability. The present study deals with the phytoplankton dynamics, biomass and physicochemical features in freshwater reservoir, Tillari, western India. The reservoir experience seasonal stratification and mixing associated changes in the biogeochemical aspects especially the phytoplankton community and chlorophyll a (hereafter, Chl a). The influence of seasonality was lesser in the deeper water in the reservoir. Buildup in phytoplankton biomass (up to 6.6 mg m−3) was observed in the upper strata of the water column (epilimnion) during the monsoon period (June–July) and winter (December) as a result of nutrient enrichment from the hypolimnion. Among nutrients, nitrate was associated with buildup of Chl a in the epilimnion during summer (r2 = 0.7). A total of 91 phytoplankton species were identified with major contribution by charophytes and chlorophytes. The dominant phytoplankton species belonged to genera Staurastrum, Cosmarium, Aulacoseira, Nephrocytium and Shroederia. Charophytes made a remarkable presence during the whole study period in the well oxygenated epilimnion as well as in the hypolimnion with relatively low oxygen. Diatom, the major silica sinking group was relatively less abundant. Keeping the importance of the reservoir in view, the understanding of phytoplankton community from this poorly explored reservoir with respect to influencing factors is a very vital baseline information. Thus, to design and evaluate the management strategies for the reservoir, continuous monitoring and processes studies is warranted.The present study deals with the phytoplankton dynamics, biomass and physicochemical features in freshwater reservoir, Tillari, western India. The reservoir experience seasonal stratification and mixing associated and changes in the biogeochemical aspects especially the phytoplankton community and chlorophyll a. Depth affected the behavior of the phytoplankton associations. Buildup in phytoplankton biomass was observed in the upper strata of the water column (epilimnion) during the monsoon period (June-July) and winter (December) as a result of nutrient enrichment from the hypolimnion. Among nutrients, nitrate was associated with buildup of Chl a during summer in the epilimnion (r2 = 0.7). A total of about 91 phytoplankton species were identified with major contribution by charophytes and chlorophytes. The dominant phytoplankton species belonged to genera Staurastrum, Cosmarium, Aulacoseira, Nephrocytium and Shroederia. Charophytes made a remarkable presence during the whole study period in the well oxygenated epilimnion as well as in the hypolimnion with relatively low oxygen. Diatom, the major silica sinking group was relatively less abundant. Keeping the importance of the reservoir in view, the understanding phytoplankton community from this poorly explored reservoir with respect to influencing factors is a very vital baseline information. Thus, to design and evaluate the management strategies for the reservoir, continuous monitoring and processes studies is warranted