Pliocene Paleoenvironment and Antarctic Ice Sheet Behavior: Evidence from Wright Valley

Investigations in Wright Valley, adjacent to the Transantarctic Mountains in East Antarctica, shed light on the question of whether high-latitude Pliocene climate was warm enough to cause widespread deglaciation of the East Antarctic craton with a concurrent Magellanic moorland-like environment. If Pliocene age diatoms, presently in glaciogenic deposits high in the Transantarctic Mountains, had come from seaways on the East Antarctic craton, an expanding Late Pliocene ice sheet must have first eroded them from marine sediments and then deposited the diatoms at their present high-altitude locations. This hypothetical expanding glacier would have had to have come through Wright Valley. Glacial drift sediments from the central Wright Valley were mapped, sampled, analyzed, and Ar-40/Ar-39 whole rock dated. Our evidence indicates that an East Antarctic outlet glacier has not expanded through Wright Valley, and hence cannot have overridden the Dry Valleys sector of the Transantarctic Mountains, any time in the past 3.8 myr. Rather, there was only moderate Pliocene expansion of local cola-based alpine glaciers and continuous cold-desert conditions in Wright Valley. Persistence of a cold-desert paleoenvironment implies that the sector of the East Antarctic Ice Sheet adjacent to Wright Valley has remained relatively stable without melting ablation zones since at least 3.8 Ma, in Early Pliocene time. A further implication is that Antarctic Ice Sheet behavior in the Pliocene was much like that in the Quaternary, when the ice sheet consisted of a stable, terrestrial core in East Antarctica and a dynamic, marine-based appendage in West Antarctica.

The Value of Adding Optics to Ecosystem Models: A Case Study

Many ecosystem models have been developed to study the ocean's biogeochemical properties, but most of these models use simple formulations to describe light penetration and spectral quality. Here, an optical model is coupled with a previously published ecosystem model that explicitly represents two phytoplankton (picoplankton and diatoms) and two zooplankton functional groups, as well as multiple nutrients and detritus. Surface ocean color fields and subsurface light fields are calculated by coupling the ecosystem model with an optical model that relates biogeochemical standing stocks with inherent optical properties (absorption, scattering); this provides input to a commercially available radiative transfer model (Ecolight). We apply this bio-optical model to the equatorial Pacific upwelling region, and find the model to be capable of reproducing many measured optical properties and key biogeochemical processes in this region. Our model results suggest that non-algal particles largely contribute to the total scattering or attenuation (> 50% at 660 nm) but have a much smaller contribution to particulate absorption (< 20% at 440 nm), while picoplankton dominate the total phytoplankton absorption (> 95% at 440 nm). These results are consistent with the field observations. In order to achieve such good agreement between data and model results, however, key model parameters, for which no field data are available, have to be constrained. Sensitivity analysis of the model results to optical parameters reveals a significant role played by colored dissolved organic matter through its influence on the quantity and quality of the ambient light. Coupling explicit optics to an ecosystem model provides advantages in generating: (1) a more accurate subsurface light-field, which is important for light sensitive biogeochemical processes such as photosynthesis and photo-oxidation, (2) additional constraints on model parameters that help to reduce uncertainties in ecosystem model simulations, and (3) model output which is comparable to basic remotely-sensed properties. In addition, the coupling of biogeochemical models and optics paves the road for future assimilation of ocean color and in-situ measured optical properties into the models.

Optimizing Models of the North Atlantic Spring Bloom Using Physical, Chemical and Bio-Optical Observations from a Lagrangian Float

The North Atlantic spring bloom is one of the main events that lead to carbon export to the deep ocean and drive oceanic uptake of CO(2) from the atmosphere. Here we use a suite of physical, bio-optical and chemical measurements made during the 2008 spring bloom to optimize and compare three different models of biological carbon export. The observations are from a Lagrangian float that operated south of Iceland from early April to late June, and were calibrated with ship-based measurements. The simplest model is representative of typical NPZD models used for the North Atlantic, while the most complex model explicitly includes diatoms and the formation of fast sinking diatom aggregates and cysts under silicate limitation. We carried out a variational optimization and error analysis for the biological parameters of all three models, and compared their ability to replicate the observations. The observations were sufficient to constrain most phytoplankton-related model parameters to accuracies of better than 15 %. However, the lack of zooplankton observations leads to large uncertainties in model parameters for grazing. The simulated vertical carbon flux at 100 m depth is similar between models and agrees well with available observations, but at 600 m the simulated flux is larger by a factor of 2.5 to 4.5 for the model with diatom aggregation. While none of the models can be formally rejected based on their misfit with the available observations, the model that includes export by diatom aggregation has a statistically significant better fit to the observations and more accurately represents the mechanisms and timing of carbon export based on observations not included in the optimization. Thus models that accurately simulate the upper 100 m do not necessarily accurately simulate export to deeper depths.

Springtime Nutrient and Phytoplankton Dynamics on Georges Bank

The dynamics of phytoplankton and nutrients before, during and after the winter-spring bloom on Georges Bank were studied on 6 monthly survey cruises from January to June 1999. We measured hydrography, phytoplankton cell densities, chlorophyll a, dissolved inorganic nutrients (NO3 + NO2, NH4, Si(OH)(4), PO4), dissolved organic nitrogen (DON) and phosphorus (DOP), particulate organic carbon (POC) and nitrogen (PON) and total particulate phosphorus (TPP). We present evidence that phytoplankton production may be significant year-round, and that the winter-spring bloom may have started in January. From January to April the phytoplankton was comprised almost exclusively of diatoms, reaching cell densities in March and April of ca. 450 cells ml(-1); chlorophyll a concentrations exceeded 10 mug l(-1) in April. Diatoms decreased to relatively low levels in May (< 50 x 10(3) cells l(-1)) and increased again in June (>300 x 10(3) cells l(-1)). Densities of dinoflagellates and nanoflagellates were low (< 10 x 10(3) cells l(-1)) from January to April, and increased in May and June to nearly 300 x 10(3) cells l(-1). Nitrate + nitrite concentrations in January were <3 muM in the shallow, central portion of the bank and decreased steadily each month. Silicate was also <3 muM over an even larger area of the central bank in January and declined to <1.5 muM over most of the Bank in April. The data suggest that silicate depletion, not DIN, contributed to the cessation of the diatom bloom. Regeneration of silicate occurred in May and June, presumably as a result of rising water temperatures in late spring which increased the dissolution rate of diatom frustules from the earlier diatom bloom. Dissolved organic nitrogen may have been utilized at the start of the winter-spring bloom; concentrations were ca, 14 muM in January, dropping to < 6 mug l(-1) in February, after which DON concentrations steadily rose to > 15 mug l(-1) in June. Overall micro-and nanoplankton biomass, measured as POC, PON and TPP, increased over the 6 mo period, as did nutritional quality of that biomass as indicated by declining C:N ratios. Our results suggest there may have been an increase in the heterotrophic component of the plankton in May and June which coincided with a second burst in diatom abundance. We discuss general features of planktonic production and nutrient dynamics with respect to year-round production on the Bank.

Modeling Responses of Diatom Productivity and Biogenic Silica Export to Iron Enrichment in the Equatorial Pacific Ocean

Using a three-dimensional physical-biogeochemical model, we have investigated the modeled responses of diatom productivity and biogenic silica export to iron enrichment in the equatorial Pacific, and compared the model simulation with in situ (IronEx II) iron fertilization results. In the eastern equatorial Pacific, an area of 540,000 km(2) was enhanced with iron by changing the photosynthetic efficiency and silicate and nitrogen uptake kinetics of phytoplankton in the model for a period of 20 days. The vertically integrated Chl a and primary production increased by about threefold 5 days after the start of the experiment, similar to that observed in the IronEx II experiment. Diatoms contribute to the initial increase of the total phytoplankton biomass, but decrease sharply after 10 days because of mesozooplankton grazing. The modeled surface nutrients (silicate and nitrate) and TCO(2) anomaly fields, obtained from the difference between the "iron addition'' and "ambient'' (without iron) concentrations, also agreed well with the IronEx II observations. The enriched patch is tracked with an inert tracer similar to the SF6 used in the IronEx II. The modeled depth-time distribution of sinking biogenic silica (BSi) indicates that it would take more than 30 days after iron injection to detect any significant BSi export out of the euphotic zone. Sensitivity studies were performed to establish the importance of fertilized patch size, duration of fertilization, and the role of mesozooplankton grazing. A larger size of the iron patch tends to produce a broader extent and longer-lasting phytoplankton blooms. Longer duration prolongs phytoplankton growth, but higher zooplankton grazing pressure prevents significant phytoplankton biomass accumulation. With the same treatment of iron fertilization in the model, lowering mesozooplankton grazing rate generates much stronger diatom bloom, but it is terminated by Si(OH)(4) limitation after the initial rapid increase. Increasing mesozooplankton grazing rate, the diatom increase due to iron addition stays at minimum level, but small phytoplankton tend to increase. The numerical model experiments demonstrate the value of ecosystem modeling for evaluating the detailed interaction between biogeochemical cycle and iron fertilization in the equatorial Pacific.

Influence of Equatorial Diatom Processes on Si Deposition and Atmospheric CO(2) Cycles at Glacial/Interglacial Timescales

The causes of the glacial cycle remain unknown, although the primary driver is changes in atmospheric CO(2), likely controlled by the biological pump and biogeochemical cycles. The two most important regions of the ocean for exchange of CO(2) with the atmosphere are the equatorial Pacific and the Southern Ocean ( SO), the former a net source and the latter a net sink under present conditions. The equatorial Pacific has been shown to be a Si(OH)(4)-limited ecosystem, a consequence of the low source Si(OH)(4) concentrations in upwelled water that has its origin in the SO. This teleconnection for nutrients between the two regions suggests an oscillatory relationship that may influence or control glacial cycles. Opal mass accumulation rate (MAR) data and delta(15)N measurements in equatorial cores are interpreted with predictions from a one- dimensional Si(OH)(4)-limited ecosystem model (CoSINE) for the equatorial Pacific. The results suggest that equatorial Pacific surface CO(2) processes are in opposite phase to that of the global atmosphere, providing a negative feedback to the glacial cycle. This negative feedback is implemented through the effect of the SO on the equatorial Si(OH)(4) supply. An alternative hypothesis, that the whole ocean becomes Si(OH)(4) poor during cooling periods, is suggested by low opal MAR in cores from both equatorial and Antarctic regions, perhaps as a result of low river input. terminations in this scenario would result from blooms of coccolithophorids triggered by low Si(OH)(4) concentrations.

Size-Fractionated Nitrogen Uptake Measurements in the Equatorial Pacific and Confirmation of the Low Si-High-Nitrate Low-Chlorophyll Condition

The equatorial Pacific Ocean is the largest natural source of CO(2) to the atmosphere, and it significantly impacts the global carbon cycle. Much of the large flux of upwelled CO(2) to the atmosphere is due to incomplete use of the available nitrate (NO(3)) and low net productivity. This high-nutrient low-chlorophyll (HNLC) condition of the equatorial upwelling zone (EUZ) has been interpreted from modeling efforts to be due to low levels of silicate ( Si( OH) 4) that limit the new production of diatoms. These ideas were incorporated into an ecosystem model, CoSINE. This model predicted production by the larger phytoplankton and the picoplankton and effects on air-sea CO(2) fluxes in the Pacific Ocean. However, there were no size-fractionated rates available for verification. Here we report the first size-fractionated new and regenerated production rates (obtained with (15)N - NO(3) and (15)N - NH(4) incubations) for the EUZ with the objective of validating the conceptual basis and functioning of the CoSINE model. Specifically, the larger phytoplankton ( with cell diameters > 5 mu m) had greater rates of new production and higher f-ratios (i.e., the proportion of NO(3) to the sum of NO(3) and NH(4) uptake) than the picoplankton that had high rates of NH(4) uptake and low f-ratios. The way that the larger primary producers are regulated in the EUZ is discussed using a continuous chemostat approach. This combines control of Si(OH)(4) production by supply rate (bottom-up) and control of growth rate ( or dilution) by grazing ( top-down control).

Physical-Biological Oceanographic Coupling Influencing Phytoplankton and Primary Production in the South China Sea

Two cruises were carried out in the summer and winter of 1998 to study coupled physical-chemical-biological processes in the South China Sea and their effects on phytoplankton stock and production. The results clearly show that the seasonal distributions of phytoplankton were closely related to the coupled processes driven by the East Asian Monsoon. Summer southwesterly monsoon induced upwelling along the China and Vietnam coasts. Several mesoscale cyclonic cold eddies and anticyclonic warm pools were identified in both seasons. In the summer, the upwelling and cold eddies, both associated with rich nutrients, low dissolved oxygen ( DO), high chlorophyll a (Chl a) and primary production ( PP), were found in the areas off the coast of central Vietnam, southeast of Hainan Island and north of the Sunda shelf, whereas in the winter they form a cold trough over the deep basin aligning from southwest to northeast. The warm pools with poor nutrients, high DO, low Chl a, and PP were found in the areas southeast of Vietnam, east of Hainan, and west of Luzon during the summer, and a northwestward warm jet from the Sulu Sea with properties similar to the warm pools was encountered during the winter. The phytoplankton stock and primary production were lower in summer due to nutrient depletion near the surface, particularly PO4. This phosphorus depletion resulted in phytoplankton species succession from diatoms to dinoflagellates and cyanophytes. A strong subsurface Chl a maximum, dominated by photosynthetic picoplankton, was found to contribute significantly to phytoplankton stocks and production.