In situ quantification of biogeochemicalprocesses at cold seeps [Elektronische Ressource] / vorgelegt von Anna Lichtschlag

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Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2009
• Nombre de lectures: 18
• Poids de l'ouvrage: 43 Mo

Extrait

In situ quantification of biogeochemical
processes at cold seeps



Dissertation
zur Erlangung des Doktorgrades
der Naturwissenschaften
-Dr. rer. nat.-



dem Fachbereich 5 (Geowissenschaften) der
Universität Bremen vorgelegt von

Anna Lichtschlag

Bremen, 2009
























1. Gutachter: Prof. Dr. Bo Barker Jørgensen
2. Gutachter: Priv. Doz. Dr. Matthias Zabel

Weitere Prüfer:
Prof. Dr. Gerhard Bohrmann
Dr. Dirk de Beer

Tag des Promotionskolloquiums: 9. November 2009
TABLE OF CONTENTS

Summary ………………………………………………………………….....……………………… 1
Zusammenfassung………………………….…………………………………………………….…. 3

Chapter 1: Cold seep systems - a general introduction
1. General introduction…………………………………………………………………….. 7
1.1. Marine cold seeps…………………………………………………………......…...……. 10
1.2. Cold seep discharges: nature, origin, and global impact………………..…………........ 14
1.3. Life at cold seeps…………………………………………………………………......… 18
1.4. Biogeochemical processes in hydrocarbon-rich sediments…………...……...…....….... 20
1.5. Cold seep mass transfer phenomena and their implication on substrate
turnover and habitat development………………………….………………..…...…...... 26
2. Objectives………………………..…………………………………………….……….. 41
3. Overview of manuscripts……………………………………….…………..….……….. 43

Chapter 2: Geochemical processes and primary productivity in different thiotrophic
mats of the Håkon Mosby Mud Volcano (Barents Sea)…………..…………...…… 59

Chapter 3: Biological and chemical sulfide oxidation in a Beggiatoa inhabited
marine sediment……………………………………………………………....……..... 95

Chapter 4: Methane and sulfide fluxes in permanent anoxia: in situ studies at
the Dvurechenskii mud volcano (Sorokin Trough, Black Sea)……...….……........ 121

Chapter 5: A deep reactive ferric iron source drives sulfide oxidation in the euxinic
surface sediments of the Dvurechenskii mud volcano (Black Sea)………...…..…. 151

Chapter 6: The H S microsensor & the dissociation constant pK : 2 1
problems & solutions………………………………………..………………...….….. 177

Chapter 7: Conclusions and perspectives………………………………………………………... 191

Danksagung…………………………………………………………………………………..……... 203
Cruise partitioning………………………………………………………………………….….....…. 204
Posters and oral presentations…………………………………………………………..……….…... 205


Summary


SUMMARY

This thesis focuses on the interaction between microbiology and geochemistry, and the physical
driving forces for the biogeochemical cycles in the near-surface sediments of cold seeps. Cold seeps
are extraordinary players in the marine realm, harboring a large biodiversity, and characterized by
intense biogeochemical reactions that are stimulated by the advective influx of reduced compounds
into the oxidized upper sediment layer (Chapter 1). Cold seeps are difficult research objects as the
sediments are highly gaseous, which makes undisturbed retrieval almost impossible, and in situ studies
at these remote habitats are only possible since a few years. The results so far show that cold seeps are
highly variable environments, both in space and time. The investigation of rates and pathways of
methane, sulfate, and sulfide turnover at cold seeps is environmentally and ecologically relevant as
methane is a powerful greenhouse gas and sulfide is toxic for most organisms, but also supports
primary production through chemoautotrophic consumption. Due to the high rates of redox processes,
chemoautotrophy is highly intense and the seeps are oases of high diverse life. They form ideal natural
laboratories to study the link between diversity and energy supply. The main objectives of this thesis
were to study the influence of transport on different biogeochemical processes and to determine which
parameters control the chemoautotrophy-driven primary production at cold seeps. In particular, it was
investigated how methane oxidation, sulfate reduction, and sulfide production are organized in cold
seeps in oxic and in permanently anoxic environments, and what impact fluid flow has on these
conversion processes. Furthermore, the pathways of sulfide oxidation in oxic and anoxic cold seep
surface sediments were studied, and the areal primary production of the different microbial habitats
was estimated. This was based on measuring the turnover of geochemically and biologically relevant
components (i.e. oxygen, sulfate, nitrate, methane, and metal-oxides), the efflux of emanating products
(i.e. sulfide and dissolved inorganic carbon), and the investigation of the distribution, physiology, and
behavior of sulfide oxidizing bacteria in situ, ex situ and experimentally.
At the Håkon Mosby Mud Volcano (Chapter 2) geochemically controlled redox reactions and
biological turnover were compared in the Beggiatoa habitat, at the gray mat site, and in the center of
the mud volcano. All three habitats showed substantial small-scale variability in carbon fixation
pathways and primary production was driven either directly through the biological use of methane or
indirectly by chemoautotrophic oxidation of sulfide derived from anaerobic oxidation of methane
(AOM). In the thiotrophic mat habitats fluxes of oxygen and nitrate were sufficient for a complete
consumption of sulfide by the microbial thiotrophic community, and geochemical sulfide oxidation
with iron-oxides was of minor importance. However, in the center of the Håkon Mosby Mud Volcano
geochemical sulfide oxidation processes predominated and biomass production was largely limited to
direct energy conservation from aerobic and anaerobic methane oxidation.
Similarly, at the coastal cold seep in Eckernförde Bay (Chapter 3) the actual importance of nitrate-
storing Beggiatoa for the benthic sulfur cycle and their success in competing with chemical sulfide
1 Summary


oxidation were studied. At Eckernförde Bay a 2-10 cm thick suboxic zone without oxygen and sulfide
was found. The highest biomass of Beggiatoa was detected within this zone, with the thiotrophs living
on the oxidation of sulfide with internally stored nitrate. Despite their high abundance, Beggiatoa-
mediated sulfide oxidation accounted only for a small fraction of the total sulfide removal in the
sediment and most of the sulfide flux into the suboxic layer was removed by geochemical processes.
Spatial variations in fluid flow as well as methane and sulfide fluxes were investigated at the
Dvurechenskii mud volcano (Chapter 4), an active mud volcano in the deep Black Sea. The aim of
this study was to determine (i) if fluid flow, methane efflux and consumption decrease radially from
the center of the mud volcano to the outer edge, (ii) if the methane consumption and sulfide production
are controlled by fluid flow, and (iii) if the mud volcano is a significant source of methane and sulfide
to the Black Sea hydrosphere. Our results showed that at the Dvurechenskii mud volcano fluid and
mud flow have significant impacts on the efflux of methane and sulfide, as well as on the anaerobic
oxidation of methane. Medium to low fluid upflow in the outer center of the mud volcano allowed
high sulfate and methane consumption and reduced the methane emission to the water column by up to
-1
70%. Instead a high fluid upflow of >1 m yr and extrusion of microbe-depleted subsurface muds, as
found at the small summit north of the geographical center, was correlated with almost no microbial
-2 -1
sulfide production and a high methane emission rate of >400 mmol m d . Our results suggest that
deep-water mud volcanoes have only a small contribution to the methane and sulfide inventory of the
Black Sea, and that most methane is derived from the abundant gas vents in shallower areas on the
Black Sea margin.
During pore fluid and solid phase analyses at the Dvurechenskii mud volcano (Chapter 5) we
found that the surface sediments of the mud volcano form a unique oxidative environment. Advecting
muds and fluids contain high concentrations of methane, but also reactive iron-minerals. In the
presence of sulfate, methane was consumed by anaerobic methane oxidation in the near-surface
sediments, and produced sulfide reacted at the surface of the AOM-zone with the iron-minerals. Thus,
sulfur intermediates such as elemental sulfur, polysulfides, thiosulfate, and sulfite formed despite the
permanently anoxic surrounding. The concentrations of these sulfur intermediates do not reach
chemical equilibrium, either due to slow kinetics or because the fluid flow continuously transports