Active pockmarks in a large lake (Lake Constance, Germany): Effects on methane distribution and turnover in the sediment

Abstract

In the eastern part of Lake Constance, the second largest prealpine lake in Europe, about 500 pockmarks (morphological depressions on the lake floor) were recently discovered. The diameters of these pockmarks are as large as 16 m, and about 24% of them continuously release methane in the form of visible bubbles. The isotopic composition of the escaping gas indicated that the methane was of biogenic origin and was predominantly produced by the CO 2 -reduction pathway. In shallow-water pockmarks (9 m and 12 m), pore-water analysis revealed increased methane concentrations of . 1100 mmol L 21 in the sediment. Diffusive methane fluxes and oxidation rates within these shallow pockmarks were much higher than in sediments outside pockmarks. An estimated methane ebullition into the water column of about 40 L h 21 per pockmark showed that methane ebullition is by far the dominant pathway of methane release from these shallow pockmarks. Meiofauna abundance and organic carbon contents of the sediment were also higher in the shallow pockmarks, but there were no differences in macrofauna. In a deep pockmark (80 m), even though bubble release was observed, the sediment inside the pockmark had almost the same methane concentrations and isotopic compositions as a site outside of the pockmark. At both sites, the shallow and deep pockmark act mainly as sedimentation traps, the emanating gas has only minor influence on the methane inventory of the sediments and, therefore, gas bubbles enter the water column essentially unaffected.

Giant lacustrine pockmarks with subaqueous groundwater discharge and subsurface sediment mobilization

“…The pockmarks described here show distinct differences in dimension, morphology, and subsurface geology to pockmarks described in other lakes. They rank among the largest and deepest known pockmarks in lakes worldwide and are similar in dimension to marine pockmarks [ Bussmann et al , ; Hovland et al , ; Hovland and Judd , ; Wessels et al , ]. The giant pockmarks in Lake Neuchâtel are characterized by distinct levee‐type sediment‐expulsion deposits, which document several phases of increased, expulsive pockmark activity.…”

“…Active mechanisms such as fluid flow from the subsurface are driving mechanism for keeping the sediment in suspension. In this geological environment, two mechanisms are plausible which commonly create and maintain pockmarks: gas flow (microbial or thermogenic) or liquid flow (e.g., groundwater) [ Bussmann et al , ; Hovland et al , ; Judd and Hovland , ]. In the following sections, we discuss our data with respect to these two hypotheses: Hypothesis I: the pockmarks are active gas seepage structures, as inferred from the amplitude anomaly within the water column, which could potentially represent a gas flare (Figure e), and Hypothesis II: they represent active groundwater seepage sites, where groundwater—possibly from the Jura Mountain karst system—flows into the lake.…”

“…The methane concentration profiles do not indicate active gas flow. However, at pockmarks originating from microbial methane seepage in Lake Constance, it was observed that sediment inside the pockmark had almost the same methane concentration as a reference site outside the pockmark, even though pockmark activity was verified by observed bubble release [ Bussmann et al , ]. Therefore, the involvement of gas seepage (microbial or thermogenic) cannot be completely excluded and may play a more important role during times of enhanced pockmark activity.…”

“…Pockmarks have been described from diverse marine geological environments [ Hovland and Judd , ; Pilcher and Argent , ]. In freshwater systems, however, reports of pockmarks are scarce [ Wessels et al , ], including only few recognized pockmark sites, e.g., as collapse structures of sublacustrine biogenic methane reservoirs [ Bussmann et al , ; Wessels et al , ], in hydrothermally active environments or near active faults [ Duck and Herbert , ; Pickrill , ; Ross et al , ], or subaquatic springs in karstic settings [ Canals et al , ; Colomer et al , ; Matter et al , ]. The latter are often hypothesized to be a significant source for groundwater supply to lake systems, which in turn have an important role as water resources.…”

Importance of allochthonous matter for profundal macrozoobenthic communities in a deep oligotrophic lake

“…The range of MOB δ 13 C used for calculations was derived from measurements of biogenic methane δ 13 C in Lake Constance (in deep profundal water layers: −40‰ to −55‰ , above littoral sediments: −70‰ ). Assuming a further fractionation of 16‰ between methane and MOB , MOB δ 13 C values may span from −56‰ to −86‰.…”