Inorganic carbon dynamics in coastal marine systems
2017-02-23T03:15:00Z (GMT) by
Carbon dioxide (CO2) plays a central role in the Earth’s climate and there is presently a great deal of interest in the exchange of this compound between atmospheric, terrestrial and marine realms. The terrestrial-marine interface is particularly dynamic and is attracting increasing interest because there is significant material deposition and recycling at this point. While there have been many studies of carbon cycling and CO2 emissions in this environment, there have been few specific studies on how anthropogenic activities affect carbon emissions and how this affects the balance between export as carbon dioxide and dissolved inorganic carbon (DIC), or carbonate alkalinity. Unlike CO2, DIC in the form of carbonate alkalinity cannot diffuse into the atmosphere, so remains dissolved. For this reason, the production of alkalinity may control the balance of inorganic carbon export to the atmosphere and to the ocean. One major source of alkalinity production in coastal systems is sulfate reduction. In order for a net alkalinity flux to occur, the product of sulfate reduction, sulfide, must be buried as iron sulfide (FeS) so that it is not reoxidised, consuming alkalinity. I therefore expect the burial of reduced solutes to play a key control over the release of inorganic carbon as CO2 (to the atmosphere) and alkalinity (exported to the ocean). This thesis examines the following 3 questions, each of which is on a different spatial scale: 1. Does alkalinity generation within tidal flat sediments control the relative export of inorganic carbon to the atmosphere and to coastal waters? 2. Do differing degrees of terrestrial inputs influence the dominant modes of carbon export from tidal flats? 3. Do differing regimes of anthropogenic land use in river catchments control inorganic carbon and alkalinity production in estuaries? In chapter 2, I investigated dissolved inorganic carbon (DIC), gaseous CO2 and total alkalinity (TA) fluxes from intertidal mudflats during periods of exposure and inundation, using laboratory core incubations, field data and reactive transport model simulations. During periods of alkalinity production, the flux of DIC out of the sediment was 1.8 times greater during inundation than exposure. The observed alkalinity production was attributed to the accumulation of reduced sulfur species within the sediment. This finding was supported by the reactive transport simulations, which showed that large amounts of sulfate reduction and subsequent reduced sulfur burial, as FeS, induced an alkalinity flux from the sediment during high tide conditions. Model simulations also found that the amount of oxidised Fe in the sediment influences the extent of net alkalinity production. Our finding, that CO2 fluxes can be significantly lower than total metabolism during exposure, has implications for studies that aim to measure metabolism on tidal flats. In chapter 3, carbon and TA export from two adjacent intertidal inlets with different terrestrial inputs were investigated. One inlet receives water from a small creek from a highly impacted, agriculturally dominated catchment, leading to the input of terrestrially sourced material; whereas, the other is relatively isolated from terrestrial inputs. The inlet with the greater amount of terrestrial inputs exported much more TA (310 vs. 46 mmol m-2 d-1), indicating that the extent of land-to-sea connectivity influences how carbon is exported from this interface. I hypothesize this is due to the increased input of iron from terrestrial sources, which fosters net TA production through the burial of reduced sulfur species as iron sulfides as found by the previous chapter. A simple mass balance showed the TA fluxes observed over 24 h were higher than could be sustained continuously with iron input from the catchment (0.49 mmol Fe m-2 d-1), indicating that the observed TA fluxes could not be sustained on long times scales by this mechanism. It is likely that there are periods of net reduction and net oxidation in response to wave action and calm conditions, highlighting the importance of long term monitoring over different seasons and weather patterns to obtain representative budgets. Keeling plots of δ13CDIC measurements over the sampling period suggested the source of DIC was from mineralisation of seagrass/microphytobenthos and mangrove organic matter. The carbon budget I produced showed that DIC was the dominant mode of carbon export from mangroves, which is relevant to recent investigations on the missing mangrove carbon sink. The 222Rn data collected during the time series measurements indicated that porewater exchange played an important part in controlling carbon export from the sediment and the residence time of porewater within both inlets was ~6.6–7.4 h, indicating that porewater exchange was driven by tidal pumping. In chapter 4, I used δ13CDIC, DIC, TA and partial pressure of carbon dioxide (pCO2) measurements to determine the dominant carbon cycling processes; aerobic respiration, anaerobic respiration (alkalinity generation) and photosynthesis in eight Southern Australian temperate estuaries. For each estuary, I calculated inorganic carbon and alkalinity budgets, and using δ13CDIC measurements, I employed a mass balance approach to determine the drivers of DIC production or consumption in the estuaries. By comparing the export of carbonate alkalinity from estuaries with varying catchment land uses, I was able to determine the differences in carbon dynamics under different levels of land use impact within the catchment. All but the least impacted estuaries showed clear non-conservative mixing behaviour of DIC, TA and δ13CDIC. The estuaries ranged between large sources and large sinks of atmospheric CO2, with fluxes from -17 – 502 mmol m-2 d-1, and were highly dependent on the sampling period. It was found that in highly impacted estuaries, there were often high rates of DIC production within the estuary (up to 510 mmol m-2 d-1) and this coincided with a high production of alkalinity(up to 273 mmol m-2 d-1). As more impacted catchments will tend to export more organic carbon and Fe, the higher rates of DIC and alkalinity production are expected owing to the burial of reduced solutes as discussed previously. Likewise, in some estuaries, high rates of photosynthesis reflect the higher loadings of inorganic nutrients from more impacted catchments. The export of alkalinity from highly impacted estuaries was highly variable between seasons. I hypothesise this is due to a higher amount of FeS being buried within sediments, which is easily oxidised when conditions permit.