Description: Microbial activity is a fundamental component of oceanic nutrient cycles. Photosynthetic microbes, collectively termed phytoplankton, are responsible for the vast majority of primary production in marine waters. The availability of nutrients in the upper ocean frequently limits the activity and abundance of these organisms. Experimental data have revealed two broad regimes of phytoplankton nutrient limitation in the modern upper ocean. Nitrogen availability tends to limit productivity throughout much of the surface low-latitude ocean, where the supply of nutrients from the subsurface is relatively slow. In contrast, iron often limits productivity where subsurface nutrient supply is enhanced, including within the main oceanic upwelling regions of the Southern Ocean and the eastern equatorial Pacific. Phosphorus, vitamins and micronutrients other than iron may also (co-)limit marine phytoplankton. The spatial patterns and importance of co-limitation, however, remain unclear. Variability in the stoichiometries of nutrient supply and biological demand are key determinants of oceanic nutrient limitation. Deciphering the mechanisms that underpin this variability, and the consequences for marine microbes, will be a challenge. But such knowledge will be crucial for accurately predicting the consequences of ongoing anthropogenic perturbations to oceanic nutrient biogeochemistry.

Description: A new model of global climate, ocean circulation, ecosystems, and biogeochemical cycling, including a fully coupled carbon cycle, is presented and evaluated. The model is consistent with multiple observational data sets from the past 50 years as well as with the observed warming of global surface air and sea temperatures during the last 150 years. It is applied to a simulation of the coming two millennia following a business-as-usual scenario of anthropogenic CO2 emissions (SRES A2 until year 2100 and subsequent linear decrease to zero until year 2300, corresponding to a total release of 5100 GtC). Atmospheric CO2 increases to a peak of more than 2000 ppmv near year 2300 (that is an airborne fraction of 72% of the emissions) followed by a gradual decline to ∼1700 ppmv at year 4000 (airborne fraction of 56%). Forty-four percent of the additional atmospheric CO2 at year 4000 is due to positive carbon cycle–climate feedbacks. Global surface air warms by ∼10°C, sea ice melts back to 10% of its current area, and the circulation of the abyssal ocean collapses. Subsurface oxygen concentrations decrease, tripling the volume of suboxic water and quadrupling the global water column denitrification. We estimate 60 ppb increase in atmospheric N2O concentrations owing to doubling of its oceanic production, leading to a weak positive feedback and contributing about 0.24°C warming at year 4000. Global ocean primary production almost doubles by year 4000. Planktonic biomass increases at high latitudes and in the subtropics whereas it decreases at midlatitudes and in the tropics. In our model, which does not account for possible direct impacts of acidification on ocean biology, production of calcium carbonate in the surface ocean doubles, further increasing surface ocean and atmospheric pCO2. This represents a new positive feedback mechanism and leads to a strengthening of the positive interaction between climate change and the carbon cycle on a multicentennial to millennial timescale. Changes in ocean biology become important for the ocean carbon uptake after year 2600, and at year 4000 they account for 320 ppmv or 22% of the atmospheric CO2 increase since the preindustrial era.

Description: It is widely recognized that the stoichiometry of nutrient elements in phytoplankton varies within the ocean. However, there are many conflicting mechanistic explanations for this variability, and it is often ignored in global biogeochemical models and carbon cycle simulations. Here we show that globally distributed particulate P:C varies as a...

Description: Measurements show that anaerobic ammonium oxidation with nitrite (anammox) is a major pathway of fixed nitrogen removal in the anoxic zones of the open ocean. Anammox requires a source of ammonium, which under anoxic conditions could be supplied by the breakdown of sinking organic matter via heterotrophic denitrification. However, at...

Description: Lake and cave records show that winter precipitation in the southwestern United States increased substantially during millennial-scale periods of Northern Hemisphere winter cooling known as Heinrich stadials. However, previous work has not produced a clear picture of the atmospheric circulation changes driving these precipitation increases. Here, we combine data with model simulations to show that maximum winter precipitation anomalies were related to an intensified subtropical jet and a deepened, southeastward-shifted Aleutian Low, which together increased atmospheric river–like transport of subtropical moisture into the western United States. The jet and Aleutian Low changes are tied to the southward displacement of the intertropical convergence zone and the accompanying intensification of the Hadley circulation in the central Pacific. These results refine our understanding of atmospheric changes accompanying Heinrich stadials and highlight the need for accurate representations of tropical-extratropical teleconnections in simulations of past and future precipitation changes in the region.