ALBERT

Description: No records exist to evaluate long-term pH dynamics in high-latitude oceans, which have the greatest probability of rapid acidification from anthropogenic CO2 emissions. We reconstructed both seasonal variability and anthropogenic change in seawater pH and temperature by using laser ablation high-resolution 2D images of stable boron isotopes (δ11B) on a long-lived coralline alga that grew continuously through the 20th century. Analyses focused on four multiannual growth segments. We show a long-term decline of 0.08 ± 0.01 pH units between the end of the 19th and 20th century, which is consistent with atmospheric CO2 records. Additionally, a strong seasonal cycle (∼0.22 pH units) is observed and interpreted as episodic annual pH increases caused by the consumption of CO2 during strong algal (kelp) growth in spring and summer. The rate of acidification intensifies from –0.006 ± 0.007 pH units per decade (between 1920s and 1960s) to –0.019 ± 0.009 pH units per decade (between 1960s and 1990s), and the episodic pH increases show a continuous shift to earlier times of the year throughout the centennial record. This is indicative of ecosystem shifts in shallow water algal productivity in this high-latitude habitat resulting from warming and acidification.

Description: Conventional integration of Earth system and ocean models can accrue considerable computational expenses, particularly for marine biogeochemical applications. "Offline" numerical schemes in which only the biogeochemical tracers are time stepped and transported using a pre-computed circulation field can substantially reduce the burden and are thus an attractive alternative. One such scheme is the "transport matrix method" (TMM), which represents tracer transport as a sequence of sparse matrix–vector products that can be performed efficiently on distributed-memory computers. While the TMM has been used for a variety of geochemical and biogeochemical studies, to date the resulting solutions have not been comprehensively assessed against their "online" counterparts. Here, we present a detailed comparison of the two. It is based on simulations of the state-of-the-art biogeochemical sub-model embedded within the widely used coarse-resolution University of Victoria Earth System Climate Model (UVic ESCM). The default, non-linear advection scheme was first replaced with a linear, third-order upwind-biased advection scheme to satisfy the linearity requirement of the TMM. Transport matrices were extracted from an equilibrium run of the physical model and subsequently used to integrate the biogeochemical model offline to equilibrium. The identical biogeochemical model was also run online. Our simulations show that offline integration introduces some bias to biogeochemical quantities through the omission of the polar filtering used in UVic ESCM and in the offline application of time-dependent forcing fields, with high latitudes showing the largest differences with respect to the online model. Differences in other regions and in the seasonality of nutrients and phytoplankton distributions are found to be relatively minor, giving confidence that the TMM is a reliable tool for offline integration of complex biogeochemical models. Moreover, while UVic ESCM is a serial code, the TMM can be run on a parallel machine with no change to the underlying biogeochemical code, thus providing orders of magnitude speed-up over the online model.

Description: Deoxygenation in the Baltic Sea endangers fish yields and favours noxious algal blooms. Yet, vertical transport processes ventilating the oxygen-deprived waters at depth and replenishing nutrient-deprived surface waters (thereby fuelling export of organic matter to depth), are not comprehensively understood. Here, we investigate the effects of the interaction between surface currents and winds (also referred to as eddy/wind effects) on upwelling in an eddy-rich general ocean circulation model of the Baltic Sea. Contrary to expectations we find that accounting for current/wind effects does inhibit the overall vertical exchange between oxygenated surface waters and oxygen-deprived water at depth. At major upwelling sites, however, as e.g. off the south coast of Sweden and Finland, the reverse holds: the interaction between topographically steered surface currents with winds blowing over the sea results in a climatological sea surface temperature cooling of 0.5 K. This implies that current/wind effects drive substantial local upwelling of cold and nutrient-replete waters.

Description: Iron is a key micronutrient for phytoplankton growth in the surface ocean. Yet the significance of volcanism for the marine biogeochemical iron-cycle is poorly constrained. Recent studies, however, suggest that offshore deposition of airborne ash from volcanic eruptions is a way to inject significant amounts of bio-available iron into the surface ocean. Volcanic ash may be transported up to several tens of kilometers high into the atmosphere during large-scale eruptions and fine ash may stay aloft for days to weeks, thereby reaching even the remotest and most iron-starved oceanic regions. Scientific ocean drilling demonstrates that volcanic ash layers and dispersed ash particles are frequently found in marine sediments and that therefore volcanic ash deposition and iron-injection into the oceans took place throughout much of the Earth's history. Natural evidence and the data now available from geochemical and biological experiments and satellite techniques suggest that volcanic ash is a so far underestimated source for iron in the surface ocean, possibly of similar importance as aeolian dust. Here we summarise the development of and the knowledge in this fairly young research field. The paper covers a wide range of chemical and biological issues and we make recommendations for future directions in these areas. The review paper may thus be helpful to improve our understanding of the role of volcanic ash for the marine biogeochemical iron-cycle, marine primary productivity and the ocean-atmosphere exchange of CO2 and other gases relevant for climate in the Earth's history.

Description: In a changing climate, marine pelagic biogeochemistry may modulate the atmospheric concentrations of climate-relevant species such as CO2 and N2O. To date, projections rely on earth system models, featuring simple pelagic biogeochemical model components, embedded into 3-D ocean circulation models. Most of these biogeochemical model components rely on the hyperbolic Michaelis–Menten (MM) formulation which specifies the limiting effect of light and nutrients on carbon assimilation by autotrophic phytoplankton. The respective MM constants, along with other model parameters, of 3-D coupled biogeochemical ocean-circulation models are usually tuned; the parameters are changed until a "reasonable" similarity to observed standing stocks is achieved. Here, we explore with twin experiments (or synthetic "observations") the demands on observations that allow for a more objective estimation of model parameters. We start with parameter retrieval experiments based on "perfect" (synthetic) observations which we distort, step by step, by low-frequency noise to approach realistic conditions. Finally, we confirm our findings with real-world observations. In summary, we find that MM constants are especially hard to constrain because even modest noise (10 %) inherent to observations may hinder the parameter retrieval already. This is of concern since the MM parameters are key to the model's sensitivity to anticipated changes in the external conditions. Furthermore, we illustrate problems caused by high-order parameter dependencies when parameter estimation is based on sparse observations of standing stocks. Somewhat counter to intuition, we find that more observational data can sometimes degrade the ability to constrain certain parameters.

Description: Anthropogenic emissions of nitrogen (N) to the atmosphere have been strongly increasing during the last century, leading to greater atmospheric N deposition to the oceans. The North Atlantic subtropical gyre (NASTG) is particularly impacted. Here, upwind sources of anthropogenic N from North American and European sources have raised atmospheric N deposition to rates comparable with N2 fixation in the gyre. However, the biogeochemical fate of the deposited N is unclear because there is no detectable accumulation in the surface waters. Most likely, deposited N accumulates in the main thermocline instead, where there is a globally unique pool of N in excess of the canonical Redfield ratio of 16 N:1 phosphorus (P). To investigate this depth zone as a sink for atmospheric N, we used a biogeochemical ocean transport model and year 2000 nutrient deposition data. We examined the maximum effects of three mechanisms that may transport excess N from the ocean surface to the main thermocline: physical transport, preferential P remineralization of sinking particles, and nutrient uptake and export by phytoplankton at higher than Redfield N:P ratios. Our results indicate that atmospheric deposition may contribute 13-19% of the annual excess N input to the main thermocline. Modeled nutrient distributions in the NASTG were comparable to observations only when non-Redfield dynamics were invoked. Preferential P remineralization could not produce realistic results on its own; if it is an important contributor to ocean biogeochemistry, it must co-occur with N2 fixation. The results suggest that: 1) the main thermocline is an important sink for anthropogenic N deposition, 2) non-Redfield surface dynamics determine the biogeochemical fate of atmospherically deposited nutrients, and 3) atmospheric N accumulation in the main thermocline has long term impacts on surface ocean biology.

Description: In the aftermath of an earthquake and tsunami on 11 March 2011 radioactive 137Cs was discharged from a damaged nuclear power plant to the sea off Fukushima Dai-ichi, Japan. Here we explore its dilution and fate with a state-of-the-art global ocean general circulation model, which is eddy-resolving in the region of interest. We find apparent consistency between our simulated circulation, estimates of 137Cs discharged ranging from 0.94 p Bq (Japanese Government, 2011) to 3.5 ± 0.7 p Bq (Tsumune et al., 2012), and measurements by Japanese authorities and the power plant operator. In contrast, our simulations are apparently inconsistent with the high 27 ± 15 p Bq discharge estimate of Bailly du Bois et al. (2012). Expressed in terms of a diffusivity we diagnose, from our simulations, an initial dilution on the shelf of 60 to 100 m2 s−1. The cross-shelf diffusivity is at 500 ± 300 m2 s−1 significantly higher and variable in time as indicated by its uncertainty. Expressed as an effective residence time of surface water on the shelf, the latter estimate transfers to 43 ± 16 days. As regards the fate of 137Cs, our simulations suggest that activities up to 4 mBq l−1 prevail in the Kuroshio-Oyashio Interfrontal Zone one year after the accident. This allows for low but detectable 0.1 to 0.3 m Bq l−1 entering the North Pacific Intermediate Water before the 137Cs signal is flushed away. The latter estimates concern the direct release to the sea only.

Description: The marine nitrogen (N) inventory is controlled by the interplay of nitrogen loss processes, here referred to as denitrification, and nitrogen source processes, primarily nitrogen fixation. The apparent stability of the marine N inventory on time scales longer than the estimated N residence time, suggests some intimate balance between N sinks and sources. Such a balance may be perceived easier to achieve when N sinks and sources occur in close spatial proximity, and some studies have interpreted observational evidence for such a proximity as indication for a stabilizing feedback processes. Using a biogeochemical ocean circulation model, we here show instead that a close spatial association of N2 fixation and denitrification can, in fact, trigger destabilizing feedbacks on the N inventory and, because of stoichiometric constrains, lead to net N losses. Contrary to current notion, a balanced N inventory requires a regional separation of N sources and sinks. This can be brought about by factors that reduce the growth of diazotrophs, such as iron, or by factors that affect the fate of the fixed nitrogen remineralization, such as dissolved organic matter dynamics. In light of our findings we suggest that spatial arrangements of N sinks and sources have to be accounted for in addition to individual rate estimates for reconstructing past, evaluating present and predicting future marine N inventory imbalances.

Description: The Beta Triangle, a region of the oligotrophic subtropical eastern North Atlantic Ocean, is notorious for its enigmatic oxygen, carbon, and nitrogen balances, in which nutrient supply is said to explain only a fraction of production necessary for estimated carbon export. Rates of dissolved organic carbon accumulation and dissolved organic nitrogen utilization in surface water and an assessment of oxygen utilized, organic matter consumed, and nitrate and phosphate regenerated in subsurface water, show that conventional production estimates miss substantial shares of biotic production. The shallow export of total organic carbon, predominantly dissolved (DOC), by subduction is responsible for about 50–70% of apparent oxygen utilization in subsurface water between the base of the surface layer at ca. 140 m and ca. 195 m depth, but it is insignificant below. Additionally, there is an estimated accumulation of 1.0 to 1.75 mol DOC m−2 a−1 in surface water. Including DOC dynamics in its carbon balance reveals the surface of this ultra-oligotrophic part of the ocean to be net autotrophic. Increasing subsurface values of excess nitrogen (DINxs) imply the export of nitrogen from surface water stemming from production not exclusively fuelled by new nitrate supplied from below. Total organic nitrogen (almost exclusively dissolved, DON) is consumed in the surface layer at a rate estimated at 0.13 to 0.23 mol m−2 a−1. There is no variation in dissolved organic phosphorus (DOP) in the same direction. DON utilization thus contributes to the pronounced subsurface DINxs signature. DOC export and accumulation are important in the carbon balance in surface and near-surface water. DON utilization and, probably, N2 fixation contribute significant amounts to the nitrogen supply of surface water. These processes can close part of the enigmatic carbon and nitrogen balances in the Beta Triangle. There are, however, no comparable processes which can explain the equally enigmatic situation concerning phosphorus supply in this area.

Description: Ocean eddies can both trigger mixing (during their formation and decay) and effectively shield water encompassed from being exchanged with ambient water (throughout their lifetimes). These antagonistic effects of eddies complicate the interpretation of synoptic snapshots typically obtained by ship-based oceanographic measurement campaigns. Here we use a coupled physical–biogeochemical model to explore biogeochemical dynamics within anticyclonic eddies in the eastern tropical South Pacific Ocean. The goal is to understand the diverse biogeochemical patterns that have been observed at the subsurface layers of the anticyclonic eddies in this region. Our model results suggest that the diverse subsurface nutrient patterns within eddies are associated with the presence of water masses of different origins at different depths.