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Sul­fur en­hances car­bon stor­age in the Black Sea

Jun 16, 2021

Study finds new explanation for the accumulation of organic compounds in oxygen-depleted marine areas.

The Black Sea is an un­usual body of wa­ter: be­low a depth of 150 metres the dis­solved oxy­gen con­cen­tra­tion sinks to around zero, mean­ing that higher life forms such as plants and an­im­als can­not ex­ist in these areas. At the same time, this semi-en­closed sea stores com­par­at­ively large amounts of or­ganic car­bon. A team of re­search­ers led by Dr. Gonzalo Gomez-Saez and Dr. Jutta Nigge­mann from the Mar­ine Geo­chem­istry Group of the Uni­versity of Olden­bur­g's In­sti­tute for Chem­istry and Bio­logy of the Mar­ine En­vir­on­ment (ICBM) and the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men has now presen­ted a new hy­po­thesis as to why or­ganic com­pounds ac­cu­mu­late in the depths of the Black Sea – and other an­oxic (oxy­gen-de­pleted) wa­ters – in the sci­entific journal Science Advances. The re­search­ers posit that re­ac­tions with hy­dro­gen sulf­ide play an im­port­ant role in sta­bil­iz­ing car­bon com­pounds. "This mech­an­ism ap­par­ently con­trib­utes to the fact that there is more than twice as much or­ganic car­bon in the wa­ters of the Black Sea as in oxy­gen-rich mar­ine areas," says Nigge­mann. "This provides a neg­at­ive feed­back in the cli­mate sys­tem that can coun­ter­act global warm­ing over geo­lo­gical peri­ods."

In the Black Sea, which cov­ers an area al­most twice the size of France, con­di­tions rarely found in other mar­ine re­gions have pre­vailed for around 7,000 years: stable strat­i­fic­a­tion largely pre­vents the mix­ing of sur­face and deep wa­ters. The wa­ter in the up­per 150 metres is low in salt and oxy­gen-rich, and comes mainly from rivers like the Danube. Be­low that is a layer of higher dens­ity sa­line wa­ter that flows into the Black Sea from the Medi­ter­ranean via the Bosporus. "When you open a wa­ter sample from the deeper areas of the Black Sea, the smell of rot­ten eggs al­most knocks you over," Nigge­mann says. On the sur­face, however, there is no in­dic­a­tion that the Black Sea is a stag­nant body of wa­ter in which, due to the lack of oxy­gen, bac­teria pro­duce foul-smelling hy­dro­gen sulf­ide.

As the new study shows, this highly re­act­ive mo­lecule binds with sub­stances from a di­verse group of car­bon­aceous ma­ter­i­als that are present in every litre of sea­wa­ter. These sub­stances are known as dis­solved or­ganic mat­ter (DOM) – a com­plex mix­ture of count­less dif­fer­ent mo­lecules that are the product of de­com­posed or­ganic mat­ter or bac­terial meta­bolic pro­cesses. "We were able to show very clearly that hy­dro­gen sulf­ide re­acts with the ex­tremely di­luted or­ganic mat­ter dir­ectly in the wa­ter," Nigge­mann ex­plains. The products of the re­ac­tion are more dur­able than the start­ing ma­ter­i­als and there­fore ac­cu­mu­late in the wa­ter.

The team com­pared wa­ter samples from dif­fer­ent loc­a­tions in the Black Sea and other seas. Us­ing vari­ous ana­lyt­ical meth­ods, in­clud­ing the ul­trahigh res­ol­u­tion mass spec­tro­meter of the ICBM-MPI Bridging Group Mar­ine Geo­chem­istry, the re­search­ers were able to char­ac­ter­ize the dis­solved or­ganic mat­ter in de­tail. They found that al­most a fifth of the or­ganic mo­lecules in the an­oxic areas of the Black Sea con­tained sul­fur – sig­ni­fic­antly more than in other seas. In ad­di­tion, the team was able to es­tab­lish that a high pro­por­tion of these com­pounds are only found in these areas, lead­ing the re­search­ers to con­clude that the sul­fur com­pounds form there through chem­ical re­ac­tions in the sulf­idic wa­ter.

Sampling device
Sampling device - a so called CTD rosette - used to obtain water samples at different depths. @ Nelli Sergeeva

Given that huge amounts of car­bon are stored in dis­solved or­ganic mat­ter – the world's oceans con­tain roughly as much dis­solved car­bon as there is CO2 in the Earth's at­mo­sphere – the res­ults of this new study are also rel­ev­ant for the cli­mate. "The volume of ocean wa­ters com­pletely de­pleted of oxy­gen quad­rupled between 1960 and 2010. Con­sequently, this sul­fur-based mech­an­ism of car­bon stor­age could in­flu­ence the chem­istry of the oceans in the fu­ture," says Gomez-Saez, the lead au­thor of the study. But this neg­at­ive feed­back is too weak to have a no­tice­able im­pact on cli­mate change un­der the cur­rent con­di­tions, he adds. In geo­lo­gical his­tory, however, there have been sev­eral peri­ods dur­ing which large areas of the oceans were oxy­gen-de­fi­cient. Dur­ing these peri­ods this mech­an­ism could have con­trib­uted to long-term re­moval of car­bon di­ox­ide from the at­mo­sphere.

The wa­ter samples from the Black Sea were taken dur­ing an ex­ped­i­tion with the re­search ves­sel Maria S. Merian. In ad­di­tion to the team from the ICBM, re­search­ers from the Al­fred We­gener In­sti­tute, Helm­holtz Centre for Po­lar and Mar­ine Re­search (AWI) in Bremer­haven, the MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences of the Uni­versity of Bre­men, and the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men par­ti­cip­ated in the study.

Research vessel RV Maria S. Merian
Research vessel RV Maria S. Merian on expedition MSM15-1, prior to departure from Istanbul to the Black Sea. © Felix Janssen

Ori­ginal pub­lic­a­tion

Gonzalo V. Gomez-Saez et al: "Sul­fur­iz­a­tion of dis­solved or­ganic mat­ter in the an­oxic wa­ter column of the Black Sea", Sci­ence Ad­vances, 7, eab­f6199.

DOI: 10.1126/sciadv.abf6199

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