1,721,018 research outputs found
Towards a mechanistic understanding of the ocean's biological carbon pump
Full text linkMarine particles lock atmospheric CO2 into organic structures that sink and export carbon into the ocean interior, a process known as the biological carbon pump (BCP). Part of this flux of particulate organic carbon (POC) is attenuated in the mesopelagic ocean as respiration, disaggregation and solubilisation wear out the sinking particles and return the organic carbon into its inorganic form. Sediment traps and imaging systems have revealed the importance that particle material composition and microstructure have in the degradation of mesopelagic POC flux. However, the data collected are scarce and have too many spatiotemporal biases to provide a mechanistic picture of which particle attributes, particle dynamics and biogeochemical factors control it. Here, I present a stochastic model of Lagrangian marine particles that mechanistically interact with each other and their biogeochemical environment as they sink through the water column. A Lagrangian framework tracks particles throughout their life history and allows recording their fractal radius, porosity, density, stickiness and sinking velocity, providing information of particle attributes alongside POC fluxes. The model is applied globally to quantify spatial variations in the vertical attenuation rate of POC flux (Martin’s b) and its dependence on proposed controlling factors such as NPP, phytoplankton community composition and seawater temperature. I find that Martin’s b is lowest in productive, diatom-rich ecosystems and highest in oligotrophic, picophytoplankton-rich ecosystems. Particle density and ballast provided by the opal sourced by diatoms are the particle attributes that contribute the most to the transfer of POC flux to the mesopelagic, and particle dynamics of coagulation and zooplankton egestion assist in spreading the opal amongst the broader pool of particles. These results emphasise the role of both surface water conditions (NPP and phytoplankton community composition) and mesopelagic conditions (particle transformation dynamics) in controlling the BCP-mediated transfer of carbon to the ocean interior
On the origin of the marine zinc–silicon correlation
Full text linkThe close linear correlation between the distributions of dissolved zinc (Zn) and silicon (Si) in seawater has puzzled chemical oceanographers since its discovery almost forty years ago, due to the apparent lack of a mechanism for coupling these two nutrient elements. Recent research has shown that such a correlation can be produced in an ocean model without any explicit coupling between Zn and Si, via the export of Zn-rich biogenic particles in the Southern Ocean, consistent with the observation of elevated Zn quotas in Southern Ocean diatoms. Here, we investigate the physical and biological mechanisms by which Southern Ocean uptake and export control the large-scale marine Zn distribution, using suites of sensitivity simulations in an ocean general circulation model (OGCM) and a box-model ensemble. These simulations focus on the sensitivity of the Zn distribution to the stoichiometry of Zn uptake relative to phosphate (PO4), drawing directly on observations in culture. Our analysis reveals that OGCM model variants that produce a well-defined step between relatively constant, high Zn:PO4 uptake ratios in the Southern Ocean and low Zn:PO4 ratios at lower latitudes fare best in reproducing the marine Zn–Si correlation at both the global and the regional Southern Ocean scale, suggesting the presence of distinct Zn-biogeochemical regimes in the high- and low-latitude oceans that may relate to differences in physiology, ecology or (micro-)nutrient status. Furthermore, a study of the systematics of both the box model and the OGCM reveals that regional Southern Ocean Zn uptake exerts control over the global Zn distribution via its modulation of the biogeochemical characteristics of the surface Southern Ocean. Specifically, model variants with elevated Southern Ocean Zn:PO4 uptake ratios produce near-complete Zn depletion in the Si-poor surface Subantarctic Zone, where upper-ocean water masses with key roles in the global oceanic circulation are formed. By setting the main preformed covariation trend within the ocean interior, the subduction of these Zn- and Si-poor water masses produces a close correlation between the Zn and Si distributions that is barely altered by their differential remineralisation during low-latitude cycling. We speculate that analogous processes in the high-latitude oceans may operate for other trace metal micronutrients as well, splitting the ocean into two fundamentally different biogeochemical, and thus biogeographic, regimes
Mechanisms driving glacial deep ocean deoxygenation
Full text linkDeep ocean deoxygenation inferred from proxies has been used to support the hypothesis that lower atmospheric carbon dioxide during glacial times was due to an increase in the strength of the ocean’s biological pump. This relies on the assumption that surface ocean oxygen (O2) is equilibrated with the atmosphere such that any O2 deficiency observed in deep waters is a result of organic matter respiration consuming O2 and producing dissolved inorganic carbon. However, this assumption has been shown to be imperfect because of disequilibrium. Here I use an Earth System Model tuned to a suite of observations, which reproduces the pattern of glacial-to-Holocene oxygenation change seen in proxy data, to show that disequilibrium plays an important role in glacial deep ocean deoxygenation. Using a novel decomposition method to track O2, I find a whole-ocean loss of 33 Pmol O2 from the preindustrial to the Last Glacial Maximum (LGM) despite a 27 Pmol gain from increased solubility due to cooler temperatures. This loss is driven by biologically-mediated O2 disequilibrium, which increases from contributing 10% of the reduction of the O2 inventory from solubility equilibrium in the preindustrial, to 27% during the LGM. Sea ice and iron fertilization are found to be the largest contributors to LGM deoxygenation, which occurs despite overall reduced production and respiration of organic matter in the glacial ocean. These results challenge the notion that deep ocean glacial deoxygenation was caused by a stronger biological pump or more sluggish circulation, and instead highlights the importance and previously under-appreciated role of O2 disequilibrium
Efficient spin-up of Earth System Models using sequence acceleration
Full text linkMarine and terrestrial biogeochemical models are key components of the Earth System Models (ESMs) used to project future environmental changes. However, their slow adjustment time also hinders effective use of ESMs because of the enormous computational resources required to integrate them to a preindustrial equilibrium. Here, a novel solution to this "spin-up" problem based on "sequence acceleration", is shown to accelerate equilibration of state-of-the-art marine biogeochemical models by over an order of magnitude. The technique can be applied in a "black box" fashion to existing models. Even under the challenging spin-up protocols used for Intergovernmental Panel on Climate Change (IPCC) simulations, the new algorithm is 5 times faster. Preliminary results suggest that terrestrial models can be similarly accelerated, enabling for the first time a quantification of major parametric uncertainties in ESMs, improved estimates of metrics such as climate sensitivity, and higher model resolution than currently feasible
Efficient spin-up of Earth System Models using sequence acceleration
No full text<p>This archive contains the scripts used to perform the simulations presented in this paper. It also contains all the data required to reproduce the figures in the paper.</p>
MITgcm 2.8deg Transport Matrix configuration
No full textThis repository contains transport matrices and associated data for the MITgcm 2.8deg configuration. For the most recent version of these files go to: http://kelvin.earth.ox.ac.uk/spk/Research/TMM/TransportMatrixConfigs/. The Transport Matrix Method driver code is available from https://github.com/samarkhatiwala/tmm
Silicon and zinc biogeochemical cycles coupled through the Southern Ocean
Full text linkZinc is vital for the physiology of oceanic phytoplankton. The striking similarity of the depth profiles of zinc to those of silicate suggests that the uptake of both elements into the opaline frustules of diatoms, and their regeneration from these frustules, should be coupled. However, the zinc content of diatom opal is negligible, and zinc is taken up into and regenerated from the organic parts of diatom cells. Thus, since opaline frustules dissolve deep in the water column while organic material is regenerated in the shallow subsurface ocean, there is little reason to expect the observed close similarity between zinc and silicate, and the dissimilarity between zinc and phosphate. Here we combine observations with simulations using a three-dimensional model of ocean circulation and biogeochemistry to show that the coupled distribution of zinc and silicate, as well as the decoupling of zinc and phosphate, can arise in the absence of mechanistic links between the uptake of zinc and silicate, and despite contrasting regeneration length scales. Our simulations indicate that the oceanic zinc distribution is, in fact, a natural result of the interaction between ocean biogeochemistry and the physical circulation through the Southern Ocean hub. Our analysis demonstrates the importance of uptake stoichiometry in controlling ocean biogeochemistry, and the utility of global-scale elemental covariation in the ocean in understanding these controls
Developing proxies in secondary carbonates with application to ENSO variability and basin-wide paleo-hydrology
Full text linkThis thesis develops new, and expands existing, methods of reconstructing paleoclimate from geochemical proxies hosted in secondary carbonate precipitates. This is done by: i) Developing a new multi-proxy approach for using lacustrine cave carbonates (LCCs) to reconstruct past shifts in precipitation-evaporation (P-E) over lake drainage basins, and ii) Investigating the suitability of speleothems as paleo-ENSO archives. Chapters 2 and 3 use modelled and measured values from LCC samples to generate a framework for understanding how Mg/Ca, U/Ca and δ44/42Ca in LCCs can be used as a combined multi-proxy approach to reconstruct past shifts in P-E. This multi-proxy approach is subsequently used to generate a novel reconstruction of P-E over the Bonneville Basin (a terminal basin in the Great Basin region of the southwestern United States), during Heinrich Stadial 1. Results suggest that moisture supply to the Great Basin region was impacted by a southwards shift of the ITCZ and variations in the elevation of North American ice sheets during Heinrich Stadial 1. Chapter 4 investigates the suitability of applying speleothem BA03 as a paleo-ENSO archive. Firstly, the impacts of age model and δ18O uncertainties on speleothem ENSO variability reconstructions are quantified. A total of eight high resolution (> 2 samples per year) δ18O time-series are then used to reconstruct shifts in ENSO variability over the Holocene. This new ENSO variability reconstruction suggests a reduction in ENSO variability during the mid-Holocene (5.68 kyr BP – 3.78 kyr BP), the termination of which coincides with the end of the so called “4.2 Kyr Event”. This thesis advances the paleoclimate application of numerous geochemical proxies, expanding the use of precisely dated, high-resolution secondary carbonate archives
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
