02653nas a2200301   4500000000100000008004100001653002500042653001800067653001800085653001200103653001600115100002000131700001900151700001800170700002100188700001900209700002400228700001900252700002100271700001700292245012700309856005200436300002000488490001200508520180200520022001402322020001502336       2012                            d10aAuthigenic carbonate10aFluid seepage10aGulf of Cadiz10amethane10aMud volcano1 aHeleen Vanneste1 aMiriam Kastner1 aRachael James1 aDouglas Connelly1 aRebecca Fisher1 aBoris Kelly-gerreyn1 aKatja Heeschen1 aMatthias Haeckel1 aRachel Mills00aAuthigenic carbonates from the Darwin Mud Volcano, Gulf of Cadiz: A record of palaeo-seepage of hydrocarbon bearing fluids  uhttp://dx.doi.org/10.1016/j.chemgeo.2012.01.006  a24\textendash390 v300-3013 a<p>Hydrocarbon-rich fluids expelled at mud volcanoes (MVs) may contribute significantly to the carbon budget of the oceans, but little is known about the long-term variation in fluid fluxes at MVs. The Darwin MV is one of more than 40 MVs located in the Gulf of Cadiz, but it is unique in that its summit is covered by a thick carbonate crust that has the potential to provide a temporal record of seepage activity. In order to test this idea, we have conducted petrographic, chemical and isotopic analyses of the carbonate crust. In addition a 1-D transport-reaction model was applied to pore fluid data to assess fluid flow and carbonate precipitation at present. The carbonate crusts mainly comprise of aragonite, with a chaotic fabric exhibiting different generations of cementation and brecciation. The crusts consist of bioclasts and lithoclasts (peloids, intraclasts and extraclasts) immersed in a micrite matrix and in a variety of cement types (microsparite, botryoidal, isopachous acicular, radial and splayed fibrous). The carbonates are moderately depleted in 13C (δ13C = - 8.1 to - 27.9\textperthousand) as are the pore fluids (δ13C = - 19.1 to - 28.7\textperthousand), which suggests that their carbon originated from the oxidation of methane and higher hydrocarbons, like the gases that seep from the MV today. The carbonate δ18O values are as high as 5.1\textperthousand, and it is most likely that the crusts formed from 18O-rich fluids derived from dehydration of clay minerals at depth. Pore fluid modelling results indicate that the Darwin MV is currently in a nearly dormant phase (seepage velocities are \&lt; 0.09 cm yr- 1). Thus, the thick carbonate crust must have formed during past episodes of high fluid flow, alternating with phases of mud extrusion and uplift.</p>  a0009-2541  a0534323936