Hengdi Liang

Hengdi Liang

Oceanographer. Modeler. Biogeochemist.

Research

Barite precipitation and dissolution mechanisms constrained in a marine barium model

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Oceanic barium (Ba) has a distribution similar to that of a nutrient, yet its distribution is thought to be regulated in large part by the precipitation and dissolution of inorganic barite (BaSO4). We have developed a global ocean model of Ba biogeochemical cycling in order to discern which processes are most important in controlling global Ba. Our approach first utilizes a combination of observations and machine learning algorithms to obtain a global Ba climatology, so that we can calculate the saturation states of barite in global oceans. We find that barite is only supersaturated in certain regions, including the surface Southern Ocean, surface Arctic Ocean, intermediate North Pacific Ocean, and intermediate Indian Ocean. Next, a mechanistic model is developed which includes both biological uptake and release of Ba, along with the precipitation and dissolution of pelagic barite, within an OCIM circulation. We find that ambient seawater is the primary source of Ba in barite, and the amount of Ba sourced from organic matter is negligible. We also tested various models for the dissolution of barite when sinking in the water column, finding that barite dissolution rates are relatively independent of the degree of barite undersaturation. Overall, this study provides new insights into the global Ba cycle, highlighting the importance of pelagic barite formation and dissolution, and providing a better mechanistic framework for the application of Ba as a paleoproductivity tracer.

Towards a better understanding of the ocean copper distribution and speciation through a data-constrained global model

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Copper (Cu) plays an essential role as a micronutrient for marine organisms, but it can also be toxic to phytoplankton when at elevated concentrations. Resolving the processes which control Cu biogeochemical cycling is therefore necessary to a complete understanding of how Cu impacts ocean life and biogeochemistry. Here we investigate the importance of various Cu cycling processes using a new global ocean Cu biogeochemical model. The model utilizes OCIM circulation and is constrained by global ocean Cu data from the GEOTRACES program. Biogeochemical processes incorporated into the model include sources from rivers, aerosol dust deposition, and sedimentary input, biological uptake and remineralization of Cu, reversible scavenging of Cu, and conversion of Cu between a labile species which is available for biological uptake and scavenging, and an inert phase of Cu which is not. The optimized model produces a good fit to global Cu observations, and highlights the key processes which control Cu distributions in the global ocean. The nearly linear increase in Cu concentrations with depth, which are observed throughout the oceans, can be achieved through either reversible scavenging or a sedimentary source, though models without either process yield a nonlinear vertical distribution. The simulated Cu in the Arctic Ocean is comparable with observational data only when there is high input to support the high surface Arctic Cu concentrations, and no or little scavenging so that Cu concentrations do not increase with depth as in other ocean basins. We find that Cu accumulates along the conveyor belt, with a higher concentration in the deep Pacific Ocean compared to the deep Atlantic Ocean, but that the relatively small concentration difference between these two ocean basins requires significantly higher external Cu inputs to the Atlantic. In addition to matching total Cu concentrations, our model is optimized to match observations of inert Cu in the North Pacific, yielding labile Cu concentrations which remain relatively constant in the water column, with increases near the seafloor, while inert Cu accumulates with the aging of water mass. We find that rivers, mineral dust, and nepheloid layer particles must each contribute with a comparable magnitude to the oceanic Cu reservoir, leading to the estimated residence time of 2,200 years.

Constraining the ocean CaCO3 export and dissolution through a global alkalinity model

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Ocean alkalinity plays a fundamental role in the apportionment of CO2 between the atmosphere and the ocean. The primary driver of the ocean’s vertical alkalinity distribution is the formation of calcium carbonate (CaCO3) by organisms at the ocean’s surface, and its dissolution at depth. This so-called “CaCO3 counterpump” is poorly constrained, however, both in terms of how much CaCO3 is exported from the surface ocean, and at what depth it dissolves. Here, we created a steady-state model of global ocean alkalinity using Ocean Circulation Inverse Model (OCIM) transport, biogeochemical cycling, and field-tested calcite and aragonite dissolution kinetics. We find that limiting CaCO3 dissolution to below the aragonite and calcite saturation horizons cannot explain excess alkalinity in the upper ocean, and that models allowing dissolution above the saturation horizons best match observations. Linking dissolution to organic matter respiration, or imposing a constant dissolution rate both produce good model fits. Our best performing models require export between 1.1 and 1.8 Gt PIC y-1 (from 73 m), but all converge to 1.0 Gt PIC y-1 export at 279 m, indicating that both high- and low-export scenarios can match observations, as long as high export is coupled to high upper ocean dissolution. These results demonstrate that dissolution is not a simple function of seawater CaCO3 saturation (Ω) and calcite or aragonite solubility, and that other mechanisms, likely related to the biology and ecology of calcifiers, must drive significant dissolution throughout the entire water column.

High precision analysis of Sn isotopes using the double spike method

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The geochemistry of tin (Sn) is poorly understood due to the difficulty to obtain accurate data of Sn concentrations and isotope ratios in geological samples. Sn isotope work can be applied to many fields of geological studies as long as the separation and precise measurement techniques are available. I conducted this project as an undergraduate student under the guidance of Prof. Weiqiang Li in Nanjing University. This project first presents how chemical separation of Sn is developed and the results of valence analysis through laser Raman spectroscopy. After column chemistry, the processed Sn solutions were analyzed using a Neptune multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). The double spike technique as well as Sb-doping and standard-sample-standard bracketing method were applied to correct instrumental mass-dependent fractionation, and high precision Sn isotope analyses were achieved; better than ±0.09‰ external precision for 122Sn/116Sn ratio was yielded. The double spike technique, which is applicable to any element with four or more isotopes, has proven to allow rigorous correction of instrumental mass fractionation to be made. Behaviors of Sb and Sn isotopes in ICP were also observed by adjusting the position of X, Y, and Z axes respectively, which proved that Sb-doping and correction is applicable for Sn isotope analysis. The effects of residual matrix elements in sample solutions on isotopic ratio measurements were evaluated, and replicate measurements of natural Sn metal, SnCl4 solutions, NIST 3161a standard Sn, and SPEX CertiPrep standard Sn yielded a long-term reproducibility.