Environmental control of sea-floor gas emissions

Methane seepage activity of NW Africa during the last deglaciation and its relation with global climate change; paleoenvironmental multiproxy analysis using foraminifera

The oce­ans oc­cu­py ∼ 71 % of the ear­t­h’s sur­face and play an im­portant role in cli­ma­te sta­bi­li­ty through the in­ter­ch­an­ge of car­bon with the at­mo­s­phe­re (Veir­ty et al., 2002). This in­ter­ac­tion has led to past chan­ges in the ear­t­h’s cli­ma­te that can help us com­pre­hend the cur­rent cli­ma­tic cri­sis our pla­net is ex­pe­ri­en­cing, of­fe­ring a de­tai­led pic­tu­re of the pos­si­ble fu­ture sce­na­ri­os thus al­lo­wing us to make con­scious de­ci­si­ons. For this re­a­son, re­se­ar­chers have been using dif­fe­rent tools and me­tho­do­lo­gies to re­con­struct past cli­ma­te chan­ges and un­der­stand the fac­tors that have trig­ge­red cri­ti­cal en­vi­ron­men­tal mo­ments in the past.

Me­tha­ne (CH₄) can be stored in two forms in ma­ri­ne se­di­ments along con­ti­nen­tal mar­gins, as ice-like gas hy­dra­te de­po­sits (Ar­cher et al., 2008) and shal­low sub­sur­face gas de­po­sits (Por­til­ho-Ra­mos et al., 2018). The­se de­po­sits are for­med un­der low tem­pe­ra­tu­re and high-pres­su­re con­di­ti­ons, and va­ria­ti­ons in such pa­ra­me­ters re­sult in the re­lease of this green­hou­se gas into the oce­an and the at­mo­s­phe­re (Kven­vol­den, 1993). Sin­ce the Last Gla­ci­al Pe­ri­od, chan­ges in glo­bal sea le­vel and tem­pe­ra­tu­re have on­set Methane seepage ac­tivi­ty in the po­les and along con­ti­nen­tal mar­gins in the Arc­tic, Pa­ci­fic and At­lan­tic oce­ans, re­leasing con­siderable amounts of me­tha­ne to oce­an wa­ters (Bi­as­toch et al., 2011; Cook et al. 2011; Ra­th­burn et al., 2000; Skar­ke et al., 2014; Por­til­ho-Ra­mos et al., 2018). Part of this green­hou­se gas is pro­ces­sed by deep oce­an bac­te­ria through an­ae­ro­bic oxi­da­ti­on of me­tha­ne (AOM) and the re­mai­ning tra­vels to shal­lo­wer sea wa­ters fi­nal­ly being re­leased to the at­mo­s­phe­re. En­han­ced AOM can re­sult in oce­an aci­di­fi­ca­ti­on and oxy­gen de­p­le­ti­on af­fec­ting lo­cal en­vi­ron­men­tal sta­bi­li­ty, whi­le the gas re­leased to the at­mo­s­phe­re cont­ri­bu­tes to glo­bal war­ming (Bi­as­toch et al., 2011).

Bent­hic fo­ra­mi­ni­fe­ra ad­apt and in­ha­bit en­vi­ron­ments with ac­tive me­tha­ne seeps, fur­ther­mo­re, cer­tain spe­cies be­co­me do­mi­nant in the­se set­tings al­beit di­ver­si­ty and po­pu­la­ti­on den­si­ty tend to stron­gly de­crea­se (Des­san­dier et al., 2019). Chan­ges in as­sem­bla­ge­s’ com­po­si­ti­ons of this mi­cro­fos­sil group re­cord past cli­ma­te chan­ges and pro­vi­de a high-re­so­lu­ti­on pic­tu­re of how tem­pe­ra­tu­re, oxy­ge­na­ti­on and tro­phic le­vels have va­ried through time and their part on glo­bal cli­ma­te re­gu­la­ti­on (Sharon et al., 2020; Ale­gret et al., 2021). For this re­a­son, their stu­dy in zo­nes af­fec­ted by me­tha­ne re­lease could pro­vi­de va­luable in­sights into how en­vi­ron­men­tal pa­ra­me­ters are af­fec­ted by me­tha­ne see­page and their in­flu­ence on glo­bal cli­ma­te chan­ge.

As methane seepage produces significant changes in environmental parameters of ocean waters before being released to the atmosphere, we intend to integrate qualitative and quantitative analysis of benthic foraminifera, with geochemical records (δ¹³C and δ¹⁸O from planktic and benthic foraminifera and Mg/Ca ratios of benthic foraminifera), to reconstruct environmental trends in the last 30.000 years and observe how methane seepage activity has influenced environmental changes in the upper slope of Mauritania (NW Africa). In this area active methane seeps have been seen (Sanz et al., 2017), offering an excellent opportunity to study this phenomenon and comprehend its influence on Global Climate stability.