IN-SITU OBSERVATION OF POCK-MARKS FROM THE NILE DEEP-SEA FAN DURING
THE NAUTINIL CRUISE: PRELIMINARY RESULTS
Lies Loncke
1*
, Germain Bayon
2
, Sébastien Duperron
3
, Jean Mascle
4
et l’équipe scientifique Nautinil
1
Université de Picardie Jules Verne, Dept de Géologie, Amiens, France- * lies.loncke@u-picardie.fr
2
IFREMER, Département Géosciences Marines, Plouzané, France - Germain.Bayon@ifremer.fr
3
IFREMER, Département Environnement Profond, Plouzané, France - Sebastien.Duperron@ifremer.fr
4
Géosciences Azur, Villefranche-sur-Mer, France - mascle@obs-vlfr.fr
Abstract
Recent geophysical surveys on the Nile deep sea fan have revealed numerous patches of high re?ectivity, mainly in the Central province,
locus of recent sediment slumps. It has been suggested that those features may correspond to pockmarks linked to gas-rich ?uids escaping
the sea?oor. Here we present preliminary results from the Nautinil cruise, which has taken place on the Nile deep-sea Fan from September
3rd to October 5th 2003. Direct observation of the sea?oor with the Nautile submersible has confirmed that these high re?ective patches
correspond to carbonate concretions associated with cold seeps. Two distinct types of structures have been identified: (1) circular
pockmarks with central carbonate chimneys or debris, and (2) massive carbonate pavements of Mg-rich calcite and aragonite. A specific
fauna has been observed in association with these seeps.
Key words: pockmarks, cold seeps, carbonate crusts, Nile deep-sea fan
Rapp. Comm. int. Mer Médit., 37,2004
48
Since 1998, several geophysical surveys (multibeam bathymetry,
backscatter imagery, 3-5 kHz profiling, MCS data) have shown that
the Nile deep sea fan (NDSF) exhibits various structures characteristic
of an active ?uid circulation within continental margin sediments.
Those structures include mud volcanoes ressembling small cones (100
to 900m in diameter), mud-pies (5 km in diameter), and pockmarks or
mounds [1; 2]. Pockmarks and mounds, represent the most widely
observed evidence of potential cold seeps on the NDSF. They are
particularly abundant in its Central province, between 1700 and 2500
water depth, where they are characterized by distinctive high
backscatter patches. In this province, a widespread field of pockmarks
coincides with an area of major sediment destabilization, which
covers an area of more than 10 000 km
2
[3]. Multibeam bathymetry
reveals that this slope area exhibits rough and chaotic small-scale
reliefs, together with linear furrows (e.g., channels), disconnecting
individual sedimentary flows. The association between those
destabilized deposits and pockmarks suggests that both phenomena
are probably closely related. Although no clear BSR has yet been
observed in this area, the potential presence of gas hydrates remains a
possibility which may explain this association between slump
deposits and pockmarks.
In september 2003, the NAUTINIL cruise, as part of the Medi?ux
European program, has allowed the in-situ observation of these highly
re?ective patches with the Nautile. Three dives on different parts of
the central province, between 1700 and 2000 meters depth, have
confirmed that high backscattered patches in this part of the Nile cone
correspond both to active and extinct cold seeps. More specifically,
high-reflectivity signals are induced by massive carbonate
concretions, which precipitate from ?uids escaping the sea?oor. Two
distinct sedimentary structureshave been observed during these
dives:
1)circular pockmarks of variable diameter(
~
3-15m), where
carbonate chimneys (up to
~
1m height) and/or infilled burrows have
sometimes built up in the central part. Observed pockmarks are
coalescent one to each other, covering areas varying from ~30m
2
to
100m
2
. Debris of authigenic carbonates and dead shells (pogomorpha
tubes, clams, urchins…) accumulate in the central part of all
pockmarks. In some pockmarks, carbonate chimneys (composed
mainly of magnesian calcite and aragonite) have been identified
together with unfilled burrows, covered by manganese oxides.
Carbonate chimneys are ~1m height and are composed of several
distinct layers of aragonite (up to 10). Unfilled burrows almost
certainly formed within the first few cm below the sediment/water
interface initially. Therefore, the presence of these burrows on top of
one carbonate chimney suggests that these chimneys may have
formed within the sediment similarly. From this above consideration,
it would be expected that freshly precipitated carbonates form from
the base of the chimney. Then, winnoying of the soft sediment around
the freshly emplaced carbonates would bring this aragonitic layer ‘out
of’the sediment and allow the chimney to “grow”. Living fauna
(pogomorpha worms, urchins) can be observed at the base of the
carbonate chimney.
2)massive carbonate pavements of aragonite, covering larger areas
(>
~
100m
2
) of the sea?oor. As a matter of fact, high-re?ectivity
signals can also be associated to massive carbonate pavements,
sometimes covered by pelagic sediments. No fauna has been observed
on their surface. Those carbonate slabs are intensively fractured in
~m
2
size pieces. This has allowed us to examine some fresh cuts
across the pavements. It consists mainly of “gruyere-like” magnesian
calcite (i.e., calcite scattered with mm- to cm-size holes). Whereas the
surface of the pavements has been intensively oxidised (i.e., brownish
ochre color), fresh cuts of aragonite are greyish. Underneath the
pavements, living fauna (pogomorpha worms, urchins, clams,
galatheas, mussels) are extremely important.
The discovery of these cold seeps associated with authigenic
carbonate precipitates demonstrate that sub-circular patches of high-
re?ectivity almost certainly correspond to areas where gas-rich ?uids
escape the sea?oor. While circular pockmarks must be linked to
localised ?uid vents, massive carbonate pavements most likely
correspond to more diffused ?ow of gas from the sea?oor.
Several push-cores have been taken and carbonate samples have
been grabbed. In the near future, a better characterization of those
authigenic carbonates (mineralogy, geochemistry, dating) will bring
additional constraints on how pockmarks may form in such cold seep
environments. In addition, geochemical analyses on sediments
sampled by the push-cores will help determining the gas source
(biogenic or thermogenic?) and the eventual presence at depth of gas
hydrates.
References
1-Loncke, L., 2002. Le delta profond du Nil: structure et évolution depuis
le Messinien (Miocène terminal). Thèse de doctorat de l’Université Paris
6. 209 p.
2-Loncke, L. Mascle, J. et l’équipe scientifique Fanil. Mud volcanoes,
gas chimneys, pock-marks and mounds in the Nile deep-sea fan (Eastern
Mediterranean): geophysical evidences. Marine and Petroleum Geology
(submitted).
3-Loncke, L., Gaullier, V., Bellaiche, G. and Mascle, J., 2002. Recent
depositional patterns of the Nile deep-sea fan from echo-character
mapping. AAPG bulletin, 86/7: 1165-1186.
