AquaDiva: Understanding the Link between the Surface and Subsurface Biogeosphere
Abstract
In the collaborative research project AquaDiva, we combine hydrogeochemical, metagemonic and biogeochemical tools to understand how the complex interactions between geologic setting and surface land use influence the function and biodiversity of the subsurface, especially ground water ecosystems. At the Hainich Critical Zone Exploratory in central Germany, we investigate soil and seepage waters in recharge areas and aquifers in a fractured limestone setting characterized by a dynamic water infiltration regime. Within the Exploratory, we have so far identified three distinct biogeochemical zones in which land use and lithologic differences combine to give rise to surprisingly different biotic communities and hydrogeochemical properties with different degrees of connection to the surface. Here we will focus on how we have combined carbon isotopic, organic biomarkers such as phospholipid fatty acids, and `omics' approaches to determine (i) how deep soil-borne microorganisms can be traced into the subsurface, and (ii) which energy sources sustain microbial life in oligotrophic limestone aquifers. With increasing travel distance to the surface, there is a decline in the abundance of microbes, with less than 5% of the taxa identified overlapping with those identified in the soils. Dissolved organic matter also is altered as it passes through soils, demonstrating an overall increase in molecular weight and a change in molecular makeup as well as radiocarbon content. Using the radiocarbon signature (corrected for the influence of carbonate dissolution), as a way to identify if organic C is being supplied recently by plants, atmosphere sources of energy for communities within the aquifer differ for the identified biogeochemical zones and include (i) dominance of inputs of fresh organic carbon from the surface feeding heterotrophy in oxygenic environments; (ii) CO2 fixation linked to nitrogen and sulfur cycling in anoxic environments and (iii) rock-derived organic matter that enters the food web and supplies up to a quarter of the carbon recycled by heterotrophic groundwater microbial communities. The degree of importance of these metabolisms, and therefore their connection to the surface is different in the three zones, with the importance of internal cycling and rock-derived organic matter as a C source surprisingly higher in anoxic aquifers. The second major goal of AquaDiva is to determine the impact of individual precipitation events on the groundwater. Initial results show dramatic effects of singular precipitation events, particularly of events during snowmelt season when the major recharge takes place. Observed large fluctuations in the water table have been linked to changes in the vertical distribution of oxygen and events of carbonate dissolution/precipitation can be identified based on variations of CO2 and O2. Ongoing research in AquaDiva seeks to better quantify the rates of groundwater movement and mixing as a way of further understanding how the emergent subsurface properties can be better linked to processes of transport and transformation within the Critical Zone.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.B44B..01T
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0486 Soils/pedology;
- BIOGEOSCIENCES;
- 0497 Wetlands;
- BIOGEOSCIENCES