Sinking phytoplankton from the surface ocean provide the principal energy source to deep-ocean ecosystems. Our aim was to understand how different phytoplankton communities impact the chemical nature of this sinking material. We show certain types of phytoplankton can preferentially export energy-rich storage compounds to the seafloor. Any climate-driven effects on phytoplankton community structure could thus impact remote deep-ocean ecosystems thousands of kilometres beneath the surface.

Streams exchange oxygen with the atmosphere, but this rate is difficult to measure. We added argon to small mountain streams to estimate gas exchange. We compared these rates with sulfur hexafluoride, an intense greenhouse gas. Argon worked well to measure gas exchange, but had higher-than-predicted rates than sulfur hexafluoride. Argon exchange is more likely to represent that for oxygen because they share similar physical properties. We suggest argon to measure gas exchange in small streams.

CO₂ outgassing from small streams causes one of the main uncertainties in global carbon budgets. These are caused by variable flow conditions, changing stream surface areas, and groundwater seeps. Here we used groundwater data to improve a novel stable carbon isotope modelling approach. We found that CO₂ outgassing contributed more than three-fourths of annual stream inorganic carbon loss in a small, silicate catchment. We underline the potential of this approach for global applications.

We measured N₂ fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. N₂ fixation rates were low but consistently detected across all depths and stations. A distinct diazotrophic phylotype dominated at 650 dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW) and suggesting that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure.

Increasing human-derived nutrient inputs to coastal oceans lead to spreading dead zones around the world. Here a biogeochemical model for the northern Gulf of Mexico, where nutrients from the Mississippi River create the largest dead zone in North American coastal waters, is used for the first time to show the effects of single and dual nutrient reductions of nitrogen (N) and phosphorus (P). Significant reductions in N or N&P load would be required to significantly reduce hypoxia in this system.

We have developed a high-resolution 3-D tomography approach and successfully applied it. We provided in situ evidence and reveal an abundance of mineral nanoparticles, in dense thin layers or nano-aggregates/clusters, instead of crystalline clay-sized minerals on or near OC surfaces. The key working minerals for C stabilization were reactive short-range-order (SRO) mineral nanoparticles and poorly crystalline submicron-sized clay minerals in a Fe oxyhydroxide-poor environment.

Soil methane (CH₄) fluxes are highly variable across natural landscapes, yet research on the variability of fluxes in unsaturated soils has not been as prevalent as in saturated portions of the landscape. In this study we measured CH₄ fluxes and environmental variables across a small mountainous watershed in central Montana. We found that CH₄ consumption in upland soils increased as the watershed became more dry and that a combination of terrain metrics can represent 47 % of the variability.

Melt ponds (MP) forming over first year ice (FYI) represent a potential source of the climate-relevant gas dimethylsulfide (DMS) to the atmosphere. Nine MP were sampled in the Canadian Arctic Archipelago. DMS concentrations reaching up to 6 nmol L⁻¹, twice the world's surface oceanic mean, were measured. Seawater intrusion appeared to seed MP with DMS-producing communities. DMS flux from Arctic MP is expected to increase in response to the expanding areal and temporal trends of MP on FYI.

We conducted a microcosm experiment on a natural phytoplankton community under year 2100 predicted CO₂ concentrations and temperature. Biomass and photosynthetic rates were significantly increased by elevated CO₂ and elevated temperature. In contrast, the combined influence of these two factors had little effect. This suggests coastal phytoplankton productivity may not be influenced by future conditions. However, the combined influence promoted the greatest diversity and increased HAB species.

There is a lack of proxies suitable for reconstructing, in a quantitative way, past changes in continental atmospheric humidity, which is a key climate parameter. Here, we demonstrate through climate chamber and climate transect calibrations that the triple oxygen isotope composition of phytoliths offers a potential for reconstructing changes in relative humidity.

To gain a better understanding of climate and marine biogeochemistry variations over the last 9500 years (the Holocene), we performed non-accelerated model simulations with a global coupled climate and biogeochemistry model forced by orbital parameters and atmospheric greenhouse gases. One main outcome is an increase in the volume of the eastern equatorial Pacific oxygen minimum zone, driven by a slowdown of the large-scale circulation.

Some calcifiers can sustain normal shell growth under hypoxia, which may involve trade-offs against shell quality. This hypothesis was tested by measuring the shell growth and shell properties of a calcifying polychaete. Despite the adverse effect on metabolism, we found that hypoxia had limited impact on shell growth probably due to the changes in shell properties as trade-offs. This suggests that such plastic response could be fundamental for calcifiers to maintain calcification under hypoxia.