Greg H. Rau

The relationship between δ13C of organic matter and [CO2(aq)] in ocean surface water: Data from a JGOFS site in the northeast Atlantic Ocean and a model

The delta 13C of suspended particulate organic matter (SPOM) in surface waters increased from -22.9 to -18.1% during April 25-May 31, 1989 at the JGOFS North Atlantic Bloom Experiment Site (NABE Site; 47 degrees N, 20 degrees W). During the same period, nearly parallel increases in sinking POM delta 13C were also found, although these values were usually lower than those of the corresponding SPOM. Consistent with the hypothesis that plankton delta 13C and [CO2 (aq)] are inversely related, the increases in both sinking and suspended POM delta 13C were highly negatively correlated with mixed-layer [CO2(aq)] that generally decreased from 13.2-10.1 micromoles/kg during the five weeks. The change in SPOM delta 13C per change in [CO2(aq)], however, appears to be somewhat greater than that expected from previous, though less direct, ocean and laboratory evidence. By adapting a model of plant delta 13C by FARQUHAR et al. (1982), it is shown that under a constant phytoplankton demand for CO2 an inverse, nonlinear SPOM delta 13C response to ambient [CO2(aq)] is expected. Such trends are unlike the negative linear relationships indicated by data from the NABE Site and or from Southern Hemisphere waters. Such differences between predicted and observed SPOM delta 13C vs. [CO2(aq)] trends and among observed relationships can be reconciled, however, if biological CO2 demand is allowed to vary. This has significant implications for the use of the delta 13C of plankton (or their organic subfractions or sedimentary remains) as a proxy for past or present ocean CO2 concentrations and biological productivity.

15N/14N variations in Cretaceous Atlantic sedimentary sequences: implication for past changes in marine nitrogen biogeochemistry

At two locations in the Atlantic Ocean (DSDP Sites 367 and 530) early to middle Cretaceous organic-carbon-rich beds (“black shales”) were found to have significantly lower δ15N values (lower15N/14N ratios) than adjacent organic-carbon-poor beds (white limestones or green claystones). While these lithologies are of marine origin, the black strata in particular have °15N values that are significantly lower than those previously found in the marine sediment record and most contemporary marine nitrogen pools. In contrast, black, organic-carbon-rich beds at a third site (DSDP Site 603) contain predominantly terrestrial organic matter and have C- and N-isotopic compositions similar to organic matter of modern terrestrial origin. The recurring15N depletion in the marine-derived Cretaceous sequences prove that the nitrogen they contain is the end result of an episodic and atypical biogeochemistry. Existing isotopic and other data indicate that the low15N relative abundance is the consequence of pelagic rather than post-depositional processes. Reduced ocean circulation, increased denitrification, and, hence, reduced euphotic zone nitrate availability may have led to Cretaceous phytoplankton assemblages that were periodically dominated by N2-fixing blue-green algae, a possible source of this sediment15N-depletion. Lack of parallel isotopic shifts in Cretaceous terrestrially-derived nitrogen (Site 603) argues that the above change in nitrogen cycling during this period did not extend beyond the marine environment.

Isotopic evidence for reduced productivity in the glacial Southern Ocean

Records of carbon and nitrogen isotopes in biogenic silica and carbon isotopes in planktonic foraminifera from deep-sea sediment cores from the Southern Ocean reveal that the primary production during the last glacial maximum was lower than Holocene productivity. These observations conflict with the hypothesis that the low atmospheric carbon dioxide concentrations were introduced by an increase in the efficiency of the high-latitude biological pump. Instead, different oceanic sectors may have had high glacial productivity, or alternative mechanisms that do not involve the biological pump must be considered as the primary cause of the low glacial atmospheric carbon dioxide concentrations.

Biological productivity during sapropel S5 formation in the Eastern Mediterranean Sea: Evidence from stable isotopes of nitrogen and carbon

We determined 15N/14N ratios in modern surface and sapropel S5 sediments of the Mediterranean Sea to clarify differences in the nutrient regime associated with sapropel formation. In the modern situation, high δ15N of unused nitrate (15–20 ‰) remaining in the surface waters during the winter phytoplankton bloom evidences P-limitation of biological production in winter. δ15N of surface sediments decrease towards the east of the basin (5 to >2.5‰). This is a consequence of either eastward increasing nitrogen fixation during the summer months, or of particulate matter being supplied predominantly by the P-limited winter bloom. Very low (−1–1‰) δ15N values in sapropel S-5 from four locations require a very light source of nutrient-N assimilated at a minimum of ten times the modern export flux. Because the isochronous records show no spatial gradient in δ15N, we exclude both Ekman-type upwelling and direct riverine discharge as likely sources of nutrients. Our data are consistent with an anti-estuarine thermohaline circulation in the upper 500m during S5 time, allowing for the trapping of nutrients in the eastern basin. The most likely scenario for S5 is that phosphorus release from a relatively shallow redox boundary resulted in an imbalanced supply of N:P (<16:1) to the photic zone. The result was a slow assimilation of carbon during summer stratification and extensive N2-fixation providing the majority of the export flux from a N-limited system.

Accelerating carbonate dissolution to sequester carbon dioxide in the ocean: Geochemical implications

Various methods have been proposed for mitigating release of anthropogenic CO2 to the atmosphere, including deep-sea injection of CO2 captured from fossil-fuel fired power plants. Here, we use a schematic model of ocean chemistry and transport to analyze the geochemical consequences of a new method for separating carbon dioxide from a waste gas stream and sequestering it in the ocean. This method involves reacting CO2-rich power-plant gases with seawater to produce a carbonic acid solution which in turn is reacted on site with carbonate mineral (e.g., limestone) to form Ca2+ and bicarbonate in solution, which can then be released and diluted in the ocean. Such a process is similar to carbonate weathering and dissolution which would have otherwise occurred naturally, but over many millennia. Relative to atmospheric release or direct ocean CO2 injection, this method would greatly expand the capacity of the ocean to store anthropogenic carbon while minimizing environmental impacts of this carbon on ocean biota. This carbonate-dissolution technique may be more cost-effective and less environmentally harmful, and than previously proposed CO2 capture and sequestration techniques.

Linking nitrogen dynamics to climate variability off central California: a 51 year record based on 15N/14N in CalCOFI zooplankton

Abstract Long-term variability in zooplankton 15N/14N was investigated in two species of calanoid copepods (Calanus pacificus and Eucalanus californicus) and two chaetognaths (Sagitta bierii and Sagitta euneritica) sampled in the spring of selected years from 1951 to 2001 off the central California coast. No statistically significant trend in 15N/14N was detected for any of the four species, with isotopic ratios in 2001 resembling those in copepods and chaetognaths sampled five decades earlier. Zooplankton body lengths also showed no long-term trends. With respect to proposed regime shifts in this region, heterogeneity in 15N/14N was detected only for S. bierii when comparing the periods 1951–1975, 1978–1998, and 1999–2001. In this species the 15N/14N in the most recent, brief period (1999–2001) averaged slightly lower than in the previous period. Three of the four species (C. pacificus, S. bierii, and S. euneritica) showed significant increases in 15N/14N during major El Ninos. El Nino-related enrichment in 15N could arise as a consequence of increased nitrate demand:supply at the base of the food web or advection of 15N-enriched nitrate from more southerly waters. While a range of physical and climate indices were evaluated, anomalies of 15N/14N from the long-term mean were found to be significantly related only to: (i) the Southern Oscillation Index in the case of both chaetognath species, (ii) a regional surface water temperature record (S. bierii only), (iii) an index of wind-driven coastal upwelling for the surface-dwelling C. pacificus, and (iv) variability in the Pacific Decadal Oscillation for the somewhat deeper-dwelling E. californicus. The relationships among each species’ 15N/14N averaged over the total sampling period was: E. californicus≈C. pacificus⪡S. euneritica

Geological setting of chemosynthetic communities in the Monterey Fan Valley system

Alvin dives and camera tows within the “meander area” of the Monterey and Ascension Fan Valleys have located nine chemosynthetic communities over depths ranging from 3000 to 3600 m over a distance of 55 km. Most of the observed communities consist largely of Calyptogena phaseoliformis, but Solemya (species unknown) and a pogonophoran (genus Polybrachia), have also been identified. The δ13C values (-35.0 to -33.6 per mil) and the presence of APS reductase and ATP sulfurylase in the C. phaseoliformis tissue is consistent with sulfur chemoautotrophy. Two reduced organic matter sources for the H2S are proposed: (1) older beds exposed by the deep erosion (up to 400 m) of the fan valleys and (2) concentrations of anaerobically decomposd organic matter buried in the valley floor.

Variations in Sedimentary Organic δ13C as a Proxy for Past Changes in Ocean and Atmospheric CO2 Concentrations

Theory, laboratory studies, and ocean observations indicate that the δ13C of marine plankton biomass and sedimentary remains thereof can be used as a proxy for ambient molecular CO2 concentration, [CO2(aq)] in ocean surface waters. A compilation of in situ ocean data suggests that about 89% of the global δ13Corg variation within bulk plankton or seston can be explained by a simple negative linear response to ambient [CO2(aq)] with the slope of the best-fit line = -0.6 ‰ µM-1. With this model the standard error of the estimate of surface ocean [CO2(aq)] is ±2.0 µM when δ13Corg is specified. This residual variability may be largely due to effects on plankton δ13Corg imparted by changes in phytoplankton CO2 demand that are independent of [CO2(aq)]. Within this variability and within the current range of ocean [CO2(aq)] there are slight differences between this model and various proposed nonlinear fits to observed global data. While an inverse relationship that can be influenced by both CO2 demand as well as concentration is theoretically expected, it does not provide an improved fit to observations over the negative linear model. When applied to the sedimentary δ13Corg record, the latter model predicts that the approximate 80 µatms increase in atmospheric pCO2 during the last glacial-interglacial transition (as documented by ice core analyses) should have resulted in a 1–2 ‰ decrease in plankton δ13C. Indeed, changes of this direction and magnitude are evident in most low-latitude Pleistocene/Holocene sediment core profiles of δ13Corg thus far reported. However, some geographic and temporal differences in past plankton isotopic response are present and expected due to i) regional non-equilibrium between ocean and atmospheric [CO2], and ii) changes in phytoplankton CO2 demand.

Predicted net efflux of radiocarbon from the ocean and increase in atmospheric radiocarbon content

Prior to changes introduced by man, production of radiocarbon (14C) in the stratosphere nearly balanced the flux of 14C from the atmosphere to the ocean and land biosphere, which in turn nearly balanced radioactive decay in these 14C reservoirs. This balance has been altered by land-use changes, fossil-fuel burning, and atmospheric nuclear detonations. Here, we use a model of the global carbon cycle to quantify these radiocarbon fluxes and make predictions about their magnitude in the future. Atmospheric nuclear detonations increased atmospheric 14C content by about 80% by the mid-1960s. Since that time, the 14C content of the atmosphere has been diminishing as this bomb radiocarbon has been entering the oceans and terrestrial biosphere. However, we predict that atmospheric 14C content will reach a minimum and start to increase within the next few years if fossil-fuel burning continues according to a “business-as-usual” scenario, even though fossil fuels are devoid of 14C. This will happen because fossil-fuel carbon diminishes the net flux of 14C from the atmosphere to the oceans and land biosphere, forcing 14C to accumulate in the atmosphere. Furthermore, the net flux of both bomb and natural 14C into the ocean are predicted to continue to slow and then, in the middle of the next century, to reverse, so that there will be a net flux of 14C from the ocean to the atmosphere. The predicted reversal of net 14C fluxes into the ocean is a further example of human impacts on the global carbon cycle.

Direct electrolytic dissolution of silicate minerals for air CO2 mitigation and carbon-negative H2 production.

We experimentally demonstrate the direct coupling of silicate mineral dissolution with saline water electrolysis and H2 production to effect significant air CO2 absorption, chemical conversion, and storage in solution. In particular, we observed as much as a 105-fold increase in OH− concentration (pH increase of up to 5.3 units) relative to experimental controls following the electrolysis of 0.25 M Na2SO4 solutions when the anode was encased in powdered silicate mineral, either wollastonite or an ultramafic mineral. After electrolysis, full equilibration of the alkalized solution with air led to a significant pH reduction and as much as a 45-fold increase in dissolved inorganic carbon concentration. This demonstrated significant spontaneous air CO2 capture, chemical conversion, and storage as a bicarbonate, predominantly as NaHCO3. The excess OH− initially formed in these experiments apparently resulted via neutralization of the anolyte acid, H2SO4, by reaction with the base mineral silicate at the anode, producing mineral sulfate and silica. This allowed the NaOH, normally generated at the cathode, to go unneutralized and to accumulate in the bulk electrolyte, ultimately reacting with atmospheric CO2 to form dissolved bicarbonate. Using nongrid or nonpeak renewable electricity, optimized systems at large scale might allow relatively high-capacity, energy-efficient (<300 kJ/mol of CO2 captured), and inexpensive (<