Browsing by Author "Monteiro, P M S"

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    Cross-shelf biogeochemical characteristics of sediments in the central Benguela and their relationship to overlying water column hypoxia
    (2007) Van der Plas, A K; Monteiro, P M S; Pascall, A
    Data from two cross-shelf sediment sampling cruises were used to explain reasons for the sediment biogeochemical variability in respect of carbon, nitrogen and sulphur, and how the cycling of these elements governs the biogeochemistry of the overlying water through their control of the redox conditions. The spatial extent of this benthic–pelagic flux link is limited to the innershelf mud belt system on the Namibian shelf. The inshore mud belt is the primary deposition area of the carbon and nitrogen new production export flux. The offshore organic-rich zones are thought to be relict particulate organic matter originating from the inshore mud belt rather than from an overlying pelagic source. These data were used to set up a multi-layer sediment model that was used through sensitivity analyses to elucidate the input characteristics that result in the most significant feedbacks on hypoxia in the overlying water. The analyses showed that, although the new production flux is a requirement to drive an oxygen demand in the sediments, the onset and persistence of anoxia may depend critically on a low-oxygen boundary condition threshold. This is thought to be a key differentiating factor between systems that, despite comparable carbon export fluxes, are characterised by a persistent hypoxia/anoxia signal and those that are characterised by episodic hypoxia events. It was concluded that sediment oxygen demand and methane and 'sulphide' emissions from the central Benguela sediments are responses to external hypoxia boundary conditions rather than the local drivers of oxygen variability.
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    Observing the seasonal cycle of pCO2 from autonomous pH measurements in the South Atlantic sector of the Southern Ocean
    (2020) Rawatlal,Mishka; Altieri, Katye E; Monteiro, P M S
    Global climate predictions hinge on our understanding of the global carbon cycle, and in particular, the role of the Southern Ocean (SO). Sea surface measurements across the SO are sparse and subject to temporal, spatial and seasonal biases. These biases arise from the inaccessibility of the SO due to the high-risk weather conditions and ice coverage experienced during the winter. This study looks at the feasibility of autonomous measuring platforms in the SO for the purpose of reducing the uncertainty bias observed in the SO, constraining the global carbon budget and observing the seasonal cycle of carbonate chemistry in seawater. The high resolution Wave Glider (WG) dataset located in the sub-Antarctic zone (SAZ) of the SO, during the spring-summer bloom period of 2013/2014, resolves the seasonal cycle of TA from pCO2 and pH (WGTA) and the empirical expression for TA (Lee et al., 2006) using salinity and temperature and measurements of pH. The discrepancy between WGTA and the calculated TA gives rise to a summer bias in the seasonal cycle of TA attributed to the uptake of nitrate during the bloom period, and the entrainment of nitrate in the pre-bloom period. The effect of this bias on estimating pCO2 indicates that the amplitude of the pCO2 seasonal cycle may be overestimated by as much as 3.6% during the pre-bloom period. An assessment of the Lee et al., (2006) TA expression (LeeTA) in the SO regime against shipboard observations showed a significant regional different in TA between the Atlantic Ocean and Indian Ocean sectors of the SO at the onset of winter, where LeeTA overestimates TA observations in the Atlantic Ocean sector. This further emphasises the seasonal bias of the TA algorithm which provides an averaged TA across the SO as a whole. Hence, it is proposed that a regional formulation be developed for the prediction of TA in each ocean sector of the SO. To further assess the performance of empirical expressions for TA, the Carter et al., (2014) LIAR expression, utilized by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project were compared to shipboard underway measurements of TA in the autumn-winter season, and WGTA. The LIAR expression showed a strong dependence on salinity that coincided with the summer bias of the Lee et al., 2006 formulation. This reenforces that estimates of TA in the SO cannot resolve biologically driven changes in the seasonal cycle of TA, and measurements of pH alone are not enough to elucidate the accurate pCO2 estimates if TA is not constrained by the seasonal cycle of nitrate.