poster 6th icgh 2008
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6th International Conference on Gas HydratesThe Fairmont Hotel VancouverVancouver, BC, CANADA July 6-10, 2008
ENVIRONMENTAL CHANGES OF THE LAST 30,000 YEARSIN THE GAS HYDRATE AREA OF JOETSU BASIN
EASTERN MARGIN OF JAPAN SEA*1,2 *2 *2 *2 *2 *2Antonio Fernando Menezes Freire , Eiichi Takeuchi , Akinori Nagasaka , Akihiro Hiruta , Osamu Ishizaki , Toshihiko Sugai*1, Ryo Matsumoto
*1 Department of Natural Environmental Studies, University of Tokyo, 524, Environmental Bldg. 5-1-5, Kashiwanoha Campus, Chiba 277-8563 Japan*2 Department of Earth and Planetary Science, University of Tokyo, 7-3-1, Hongo Campus, Bunkyo-ku, Tokyo 113-0033 - Japan
GAS HYDRATE STUDY AREA
Fig. 02: Map of the study area showing Joetsu Knoll andUmitaka Spur gas hydrate areas. Also this map shows piston coring location.
MAIN PURPOSESA) To understand the sedimentar history of the Late Quaternary using the stratigraphic and geochemical records from piston- cores collected on a gas hydrate area located on the Eastern Margin of Japan Sea, south of the Sado Islands (Figs. 01 and 02)B) To make a correlation between these records on Japan Sea and those observed on the drilling core CK-06 on the Eastern Margin of the Pacifc Ocean, east of Shimokita Peninsula (Fig. 01).C) To infer the methane flux variations along the geologic time using geochemical data.
ABSTRACTRecently, we recognized active methane venting and gas hydrates, which are widely distributed on just belowthe sea floor in the Joestu basin, eastern margin of Japan Sea. This study has the intention to give supportfor future works, understanding the Late Quaternary history of the study area. Interbedded dark gray thinly laminites and dark brown to gray bioturbated units are common throughout theQuaternary sediments of Japan Sea, and have been often explained in terms of glacio-eustatic sea-level changes. These layers have a very good correlation because they occur in all Japan Sea. We used totalorganic carbon (TOC) content and carbon isotopic composition of the gas hydrates bearing-sediments in order to identify the nature of the organic matters present in the study area and to make a correlation withsamples collected in the Pacific Ocean. Associated with XRD analysis, these data helped us to locate theHolocene/Pleistocene boundary, to identify key stratigraphic surfaces, and to recognize sulfate-methane interfaces. Different SMI occurs due methane flux variation with the geologic time.
13TOTAL ORGANIC CARBON AND d C CONCENTRATIONSThe Holocene/Pleistocene BoundaryPiston cores data from the study area shows a clear upward increasing of both TOC and
13d C contents. It is possible to identify a shift on TOC curve, from 1.3% to 2.0% and from 13-26‰ to -22‰ on d C curve, around 3.5 m depth in reference piston core PC-701 graph
(Figure 03a). The same increased pattern can be observed in piston cores PC-702(Figure 03b) and PC-706 (Figure 03c), respectively located over Joetsu Knoll and UmitakaSpur gas hydrate areas, but at different depths. These shift depths represent importantchanges on environmental conditions in the study area: Below occurs a relative small TOC
13production with d C values around -26‰ and, above this shift, occurs a higher TOC 13production with heavier d C isotopic composition.
13CK-06 and GISP-2 (Figure 04) TOC and d C profiles have very similar pattern and are very easily correlated with Japan Sea study area. The conclusion is that both Japan Seaand Pacific Ocean had the same changes on organic matter production, and the boundary Holocene/Pleistocene is very well marked using this criteria.
THE NATURE OF ORGANIC MATTER: MARINE vs. TERRESTRIAL13TOC and d C content indicate the origin and intensity of organic matter production. The warmer of the sea water and the rise
of the sea level during Holocene induced the free communication between Japan Sea and the Pacific Ocean because the straits are more deep and large. Because this condition, more warm-water species of diatom and radiolaria became more
12present at Japan Sea [Oba et al. 1991]. As organic matter, generated by plankton, removes C selectively from the surface
13 13water, the d C of planktonic foraminiferal tests becomes heavier. According Burdige [2006], d C of TOC, in association with nitrogen content can potentially be used to differentiate sources of organic matter. Organic matter produced by phytoplankton
13has very different d C values from that produced by land plants because of differences in the isotopic composition of their 13carbon source [Burdige, 2006]. The primary carbon source for marine phytoplankton is seawater bicarbonate, with a d C of
13~0‰. In contrast, land plants use atmospheric CO as their carbon source, with d C of around -7‰ [Burdige, 2006]. Differences2
in the mechanisms of CO uptake by terrestrial plants (CO diffusion) versus marine plants (active uptake of bicarbonate in most2 2
cases) also lead to some additional amounts of carbon fractionation during photosynthesis. As a result of all of these factors, 13marine organic matter generally has a d C of around -17‰ to -22‰ and terrestrial organic matter of around -25‰ to -28‰
[Burdige, 2006].Marine organic matter generally has a C/N ratio between 5 and 10 and fresh terrestrial organic matter has a C/N ratio >20. These differences are a result of dissimilarities in the structural components of marine versus terrestrial plants: carbon-richlignocelluloses in the latter and nitrogen-containing proteins in the former. While the Holocene is characterized by a higher marine organic matter production, Pleistocene is characterized by lower
13organic matter production with lighter d C isotopic composition due to terrestrial organic matter source. The anoxic layerTL-2 in this time are due the lower water circulation because sea level drops on LGM. During this time, when the sea level was120m below present sea level, the coastal line was very close present shelf broken and the shallow and narrow straits (Figure05) reduced drastically the sea water circulation causing clay minerals and organic matter to stay more time at suspensioninducing strong anoxic conditions and the occurrence of dark-gray thin laminated mud. The crossplot with data from both CK-06and UT-07 cruise shows three groups of organic matter values (Figure 06a) and, according Burdige [2006] criteria, it is possibleto infer the nature of organic matter production at each geologic time. Comparing to Japan Sea study area, the samples fromPacific Ocean have more relation with marine production. Crossplot with samples only from Japan Sea study area (Figure 06b)shows that, excepting PC-701, organic matter has a terrestrial or mixing source. Umitaka Spur and Joetsu Knoll (Figures 02and 05) are closer to shore line than PC-701 and, at Pleistocene, the “bay” stage induced weak circulation.
Fig. 05: Coastal and bathymetric map of Joetsu basin, south of Sado Islands frompresent andduring the LGM time, when the sea level dropped around 120m than present sea level. Thenarrow and shallow strait between Sado Islands and the western coast of Japan limited thecirculation of sea water currents making Joetsu basin like a big bay.
13 13Fig. 06:a) Crossplot TOC x d C data from CK-06 (crosses) and UT-07 (squares). Three groups can be seen: relative higher TOC values and d C heavier than ~-22‰ 13 13(marine phytoplankton production); relative medium TOC and d C between ~-22‰ and ~-25‰ (mixed or non determinate); and relative lower TOC and d C lighter
13than ~-25‰ (vascular land plants). According Burdige, 2006. b) Crossplot TOC x d C data from UT-07 samples. PC-701, located far from the coastal line and into atypically depositional site, shows a large range of values and indicate both terrestrial and marine organic matter source. The other cores have a small range between terrestrial to mixed organic matter, according Burdige [2006].
FIGURE XX
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CK-06(SHIMOKITA PENINSULA )
UT-07/NT-07-20
PC-701
CORE LOCATIONS
JAPAN SEA
PACIFIC OCEAN
Fig. 01: Core location map. UT-07 (Umitaka Maru) cruise and NT-07-20 (Natsushima Cruise) are located over Umitaka Spur and Joetsu Knoll gas hydrate areas. PC-701 is a reference site and belongs to UT-07cruise. CK-06 (Chikyo) cruise is located at Pacific Ocean and is a refence to make correlation.
CORE LOCATIONS MAP
d
d
d
d
d
TL-2
TL-1
d
Fig. 03c: PC-706 located over Umitaka Spur gas hydratesite. Curves show the same pattern.
TL-1
TL-2
d
Fig. 03b: PC-702 located over Joetsu Knoll gas hydrate 13site.The same pattern can be seen in both TOC and d C
and both thin laminated -1 (TL-1) and TL-2 are present. 13Fig. 03a: PC-701 reference piston core. Both TOC and d C curves show ashift around 3.5m and it represents changes on organic matter production.
This depth is here interpretated like the boundary between Holocene and14 fPleistocene and it is confirmed by C dating from foramini era tests and
the occurrence of AT tephra layer (~30Ka).
TL-1
TL-2
d
AT ash layer (~30Ka)
14Foraminifera C (19.6~15.8Ka)
14Foraminifera C (10.9~7.4Ka)
V v v v v
Fig. 04: Correlation between CK-06 drilling core and GISP-2 ice core. The same increased pattern 13 14on both TOC and d C curves, associated with foraminifera C dating indicate the boundary
18Holocene/Pleistocene. The d O curve from GISP-2 indicates warmer conditions at Holocenetime and the TOC curve shows that a more effective organic matter production occurs. Also,
13the d C curve indicate heavier isotope compositions and, according Burdige, 2006, it reflects a marine organic matter production. Pacific Ocean curves are very easily correlated with Japan Seacurves and both have the same signature.
TERRIGENOUS MATERIAL INPUT13The boundary between the Holocene and Pleistocene could be marked by TOC and d C isotopic
concentration how discussed before but, also, this boundary could be identified using clay minerals,quartz and feldspars content (Figures 08a and 08b). During the LGM, eustatic sea level lowering 120mbelow present sea level and would have severely restricted or completely blocked the inflow into the study area [Oba et al. 1991] (Figure 05). Because of this sea level dropping, the river`s mouths were very close to the edge of the shelf and the discharge form ice melting with sediments in suspension occurred directly on the slope. In particular, Joetsu basin coastal line was more than 20km sea ward during the LGM (Figure 05), and a very shallow and narrow strait was formed between the Sado Islands and the western Japan coastal line. The sea water flow through this pathway was very low and Joestu basin was a big bay. The poor sea water circulation could not spread fine grain floated sediments and it stays at suspension for more time. Little by little, clay minerals sunk to the sea floor. On the other hand, at the Holocene, the sea level rising induced a good circulation and clay minerals were washed over. At the same time, the increasing of the weathering because to the melt of ice in response of warmer climate, induced quartz and feldspars transportation by rivers and rapidly precipitate to the sea floor. Quartz and feldspars have no porosity while clay minerals have a lot of porous. Because this, quartz and feldspars can not float for a long time and rapidly sink to the sea floor. The lower clay minerals content at Pleistocene coincides with the lower TOC content (Figure 03a). It is because LGM time, when the weather was very cold and dry and, also, river’s mouth were more far from due low sea level condition.
6th International Conference on Gas HydratesThe Fairmont Hotel VancouverVancouver, BC, CANADA July 6-10, 2008
SULFATE OXIDATION OF METHANE
Sea water and sediment pore water have a lot of ions dissolved. The sediment particles also have cations andanions adsorbed mainly on clay minerals. When a methane flux comes to the sea floor, both thermogenic or
2- 2-biogenic origin, an oxidation of methane occurs (Figure 10). So , Co and H S are not stable and the presence4 3 2
of cations dissolved in the interstitial water react with them and forming sulfates, carbonates and sulfides. Barite,calcaite, aragonite, dolomite and pyrite are commom authigenic minerals that precipitate around the sulfate-methane interface (SMI).Samples collected from UT-07 cruise shows some “fronts” of barite, calcite and pyrite (Figures 11, 12 and 13). Instead calcite and toral inorganic carbon (TIC) values can have association with foraminifera, sulfur, pyrite andbarite have no relation with them and are good indicators of SMI. Because methane flux can vary with time, SMI can be shallower or deeper accoding the flux intensity. Depending on the time that SMI is stable at the same depth, the reaction will be more effective inducing more chemical precipitation. On the other hand, if methane flux oscillates very quickly, the reaction is not so strong and a poor amount of authigenic minerals will be formed.
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ENVIRONMENTAL CHANGES OF THE LAST 30,000 YEARSIN THE GAS HYDRATE AREA OF JOETSU BASIN
EASTERN MARGIN OF JAPAN SEA*1,2 *2 *2 *2 *2 *2Antonio Fernando Menezes Freire , Eiichi Takeuchi , Akinori Nagasaka , Akihiro Hiruta , Osamu Ishizaki , Toshihiko Sugai*1, Ryo Matsumoto
*1 Department of Natural Environmental Studies, University of Tokyo, 524, Environmental Bldg. 5-1-5, Kashiwanoha Campus, Chiba 277-8563 Japan*2 Department of Earth and Planetary Science, University of Tokyo, 7-3-1, Hongo Campus, Bunkyo-ku, Tokyo 113-0033 - Japan
AKNOWLEDGEMENTSFor our colleagues on both Department of Earth and Planetary Science and Department ofNatural Environmental Studies that help us on analysis, discussions and other supports.
Figure 10. Diagarm about sulfate-oxidation of methane oxidation and the formation of the sulfate-methane interface (SMI).
Figure 08. PC-701 clay minerals (a) and quartz, feldspars and quartz/feldspars ratio profiles. The boundary between the 13Holocene and Pleistocene could be marked by TOC and d C isotopic concentration how discussed before but, also, this
boundary could be identified using clay minerals, quartz and feldspars content.
vvvv
Figure 11. PC-701 SMI profile. TIC, calcite, barite, pyrire and sulfur curves show peaks at similar depths. Note that the present SMI is locatedat the depth where SO content is near zero and CH becomes high. A strong coincidence with this SMI with chemical peaks indicates4 4
that it is agood parameter to identify SMI. TIC and calcite can have influence of foraminifera, but barite, pyrite and sulfur have no contaminationand can calibrate the data. Peaks above and below indicate fossil SMI, when methane flux was stronger (upper) and weaker (lower). This location
13is a refence site and no evidence about gas hydrate was found at this place. Instead of this, methane flux is present and its d C around -87‰indicates biogenic origin.
Figure 12. PC-702 SMI profile. This is a gas hydrate site located over Joetsu Knoll. Plumes and gas hydrate are present and were recovered13and analysed. Also, gas chymineis and faults have been see on seismic data. A d C around -50‰ indicates mixed origin. Note that present SMI
is shallower than at PC-701, indicating that methane flux over Joetsu Knoll is stronger than at reference site.
Figure 13. PC-707 SMI profile. Located over Umitaka Spur gas hydrate site, this piston core shows a very shallow present SMI. The samefeatures occurred at Joetsu Knoll are present here and shallower positioning of present SMI and no occurrence of the upper fossil SMI indicatethat methane flux is now stronger than at Joetsu Knoll. An erosion can be occurred and cut the upper SMI. High values of pyrite and sulfur nearsea floor sugest erosion because the sea floor is predominatily oxidized.
CONCLUSIONS
The late Quaternary correlation between Japan Sea and the Pacific Ocean is possible using piston-13
coring data. TOC and d C increased pattern is very similar in both sites. It indicates more organic 13
matter production during Holocene and the d C increased pattern upward suggests a phytoplanktonorganic matter production.
The poor sea water circulation at Pleistocene, due to the drop of sea level at LGM, caused a notgood spreading of clay minerals, and, little by little, it was sinking to the sea bottom. At Holocene,the rising of the sea level induced a good sea water circulation on Japan Sea and clay minerals were easily washed over seaward. At the same time, the climate warm increasing induced the snowmelt on the mountains located near the shoreline of Niigata Prefecture, causing the increasing ofweathering. Because this, quartz and feldspars were delivered by rivers, arriving to Joetsu Basin and sinking to sea floor faster than clay minerals.
Geochemical records of sulfate-oxidation of methane is present by several peaks of calcite, barite,pyrite and sulfur. At least two sets of peaks are present and represent different stages of the sulfatemethane interface (SMI). Present SMI and fossil SMI can be infered and it can infer that the fluxof methane was not constant withe the geologic time. The peaks above and below present SMIindicates that methane flux was stronger (upper) and weaker (lower) than present level.
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