Ex­plan­a­tion for un­usual iso­tope pat­terns

In a labor­at­ory ex­per­i­ment, re­search­ers sim­u­late al­tern­at­ive hy­dro­car­bon form­a­tion through re­duc­tion of acetic acid

15-Nov-2021 - Germany

Hy­dro­car­bons, which are an es­sen­tial com­pon­ent of crude oil and nat­ural gas, form un­der pres­sure and high tem­per­at­ures in the deep ocean floor. In the Guay­mas Basin in the Gulf of Cali­for­nia, re­search­ers have de­tec­ted hy­dro­car­bon gas pat­terns that could not have been gen­er­ated by known form­a­tion path­ways. In their study, which has now been pub­lished in the journal Proceedings of the National Academy of Sciences (PNAS), they de­scribe a new path­way for the form­a­tion of two of the main com­pon­ents of nat­ural gas, eth­ane and pro­pane, via re­duc­tion of acetic acid. The team, com­pris­ing re­search­ers from MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences at the Uni­versity of Bre­men and the Uni­versity of North Car­o­lina (USA), has in­vest­ig­ated the iso­topic sig­na­tures and sim­u­lated hy­dro­car­bon form­a­tion in labor­at­ory ex­per­i­ments.

Min Song

The manned submersible vehicle Alvin of the Woods Hole Oceanographic Institution (WHOI) is deployed from the deck of the Research Vessel ATLANTIS.

The iso­topic com­pos­i­tions of hy­dro­car­bon com­pounds are like a fin­ger­print. They can clearly in­dic­ate the way in which hy­dro­car­bons like meth­ane, eth­ane, pro­pane, bu­tane and pentane are formed. When hy­dro­car­bons are com­bus­ted, wa­ter and car­bon di­ox­ide are pro­duced and en­ergy is re­leased. Hy­dro­car­bons, in­clud­ing crude oil and nat­ural gas, are formed over geo­lo­gical time peri­ods un­der high tem­per­at­ures and pres­sures, and re­search­ers can ul­ti­mately identify this pro­cess based on its par­tic­u­lar iso­tope pat­tern.

While study­ing samples from the Guay­mas Basin, however, re­search­ers de­tec­ted iso­tope pat­terns that did not fit with the well-known form­a­tion path­ways. The samples were col­lec­ted us­ing the manned sub­mers­ible vehicle Alvin dur­ing sev­eral ex­ped­i­tions with the re­search ves­sel AT­LANTIS, so it was pos­sible to pre­cisely define the sampling loc­a­tions and at the same time meas­ure the tem­per­at­ure at the ocean floor. “In the Guay­mas Basin, it is pos­sible to wit­ness oil form­a­tion as though it were in time-lapse,” ex­plains co-au­thor Dr. Florence Schubotz of MARUM. Large quant­it­ies of or­ganic ma­ter­ial are de­pos­ited as sed­i­ments in this basin, and these, in turn, are rap­idly heated by the hy­dro­thermal activ­ity, which res­ults in oil form­a­tion. This oc­curs very quickly com­pared to the mil­lions of years needed for con­ven­tional oil form­a­tion at high pres­sure and tem­per­at­ure. In con­trast to that pro­cess, the geo­lo­gical activ­ity on the con­tin­ental mar­gin in the Guay­mas Basin and the quant­it­ies of bio­lo­gical ma­ter­ial de­pos­ited are the primary factors in the gen­er­a­tion of hy­dro­car­bons. Thus the basin op­er­ates like a pres­sure cooker. Be­cause of this unique char­ac­ter, the Guay­mas Basin is con­sidered by re­search­ers to be a model re­gion for the rapid thermal al­ter­a­tion of sed­i­ment­ary or­ganic mat­ter.

“These data sur­prised us, as we could not ex­plain them with any known mech­an­isms. We brain­stormed al­tern­at­ive ex­plan­a­tions, con­sid­er­ing what makes this sys­tem so spe­cial and what pro­cesses might take place in the deep sub­sur­face that could have caused the un­usual iso­tope pat­tern of hy­dro­car­bon gases in our samples,” ex­plains first au­thor Dr. Min Song. In do­ing this, the re­search­ers con­sidered the role of volat­ile fatty acids, which are abund­ant in the Guay­mas Basin sub­sur­face, and con­duc­ted sim­u­la­tion ex­per­i­ments. For the first time, they have pro­posed an al­tern­at­ive path­way for the form­a­tion of the gases, which also ex­plains the iso­tope sig­na­tures ob­served in the basin’s sub­sur­face. Tem­per­at­ures and pres­sures like those ex­ist­ing at hy­dro­thermal sys­tems were sim­u­lated in the labor­at­ory of MARUM re­searcher Prof. Wolfgang Bach. Eth­ane and pro­pane were gen­er­ated from acetic acid without the in­flu­ence of mi­croor­gan­isms.

Schubotz points out that this new ex­plan­a­tion for an al­tern­at­ive form­a­tion pro­cess for hy­dro­car­bons in the ocean floor can now be tested in other geo­therm­ally and hy­dro­therm­ally heated sed­i­ment sys­tems. For this reason, the res­ults are an im­port­ant con­tri­bu­tion to the re­search at MARUM. Min Song’s col­leagues in the Cluster of Ex­cel­lence “The Ocean Floor – Earth’s Un­charted In­ter­face” are in­volved in the study as well as in­ter­na­tional part­ners, in­clud­ing Prof. An­dreas Teske of the Uni­versity of North Car­o­lina, Chapel Hill. “Our find­ings provide a con­vin­cing ex­plan­a­tion for the un­usual iso­tope pat­terns in hy­dro­car­bon gases," says co-cluster spokes­per­son and study leader Prof. Kai-Uwe Hin­richs. “The hy­dro­car­bons here are not formed by split­ting longer com­pounds into smal­ler com­pon­ents, but in­stead are gen­er­ated from smal­ler build­ing blocks.” No mi­croor­gan­isms are in­volved in this form­a­tion pro­cess, which is why this pro­cess falls into the cat­egory of abi­otic form­a­tion. The find­ings will lead to a bet­ter un­der­stand­ing of the pro­cesses in the ocean floor, par­tic­u­larly with re­gard to the flow of car­bon.

Original publication

Min Song, Florence Schubotz, Matthias Y. Kellermann, Christian T. Hansen, Wolfgang Bach, Andreas P. Teske, Kai-Uwe Hinrichs; "Formation of ethane and propane via abiotic reductive conversion of acetic acid in hydrothermal sediments"; PNAS; 2021

Other news from the department science

Most read news

More news from our other portals