New method helps un­der­stand the global or­ganic car­bon cycle

Re­search­ers from the USA, the UK and Bre­men have used a new method to cal­cu­late the rate at which or­ganic car­bon was se­questered in mar­ine sed­i­ments dur­ing the Earth’s Neo­gene period. It is more ac­cur­ate than the pre­vi­ously used method and can there­fore make an im­port­ant con­tri­bu­tion to un­der­stand­ing cli­mate change and mit­ig­at­ing its ef­fects. Their res­ults have now been pub­lished in the sci­entific journal Nature.

Oceans serve as a sink for car­bon. This means that car­bon from the at­mo­sphere is stored in the oceans. However, a dis­tinc­tion is made between or­ganic and in­or­ganic car­bon. The or­ganic car­bon bound in mar­ine sed­i­ments is also a source of oxy­gen. Un­til now, it has been com­mon prac­tice to de­term­ine the mass bal­ance between in­or­ganic and or­ganic car­bon – but this method is con­sidered in­ac­cur­ate. A team from the USA, Great Bri­tain and Ger­many has now de­veloped a dif­fer­ent ap­proach. Their goal was to be able to bet­ter de­term­ine the rate of car­bon over a longer period of time.

To do this, they used data from deep-sea drilling at 81 global sites to de­term­ine the his­tory of or­ganic car­bon burial dur­ing the Neo­gene (about 23 to 3 mil­lion years ago). This ap­proach makes it pos­sible to bet­ter de­term­ine the vari­ab­il­ity over such a long period of time.

”Our res­ults sup­port the as­sump­tion that rates were high in the early Mio­cene and Plio­cene and low in the middle Mio­cene,” ex­plains first au­thor Dr. Ziye Li from MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences at the Uni­versity of Bre­men. “We cal­cu­lated the mass ac­cu­mu­la­tion rate of or­ganic car­bon dir­ectly from the or­ganic car­bon con­tent of mar­ine sed­i­ments. This is pos­sible thanks to stand­ard­ized meas­ure­ments com­bined with well val­id­ated age mod­els from sites from the in­ter­na­tional drilling pro­gram IODP and its pre­de­cessors DSDP and ODP. Tra­di­tion­ally, es­tim­ates have been based on the iso­topic com­pos­i­tion of car­bon, which re­quires, among other things, a num­ber of as­sump­tions about car­bon sources and key fluxes within the car­bon cycle,” says Li, who works at MARUM in the Low Lat­it­ude Cli­mate Vari­ab­il­ity group.

“Our new res­ults are very dif­fer­ent – they are the op­pos­ite of what the iso­tope cal­cu­la­tions are sug­gest­ing,” says co-au­thor Ben­jamin Mills from the Uni­versity of Leeds, an ex­pert on the es­tab­lished iso­tope meth­ods. “I was really sur­prised how wrong our cur­rent ideas might be.”

Li and her col­leagues as­sume that car­bon se­quest­ra­tion, or rather its ab­sence, is re­lated to tem­per­at­ure-de­pend­ent bac­terial de­com­pos­i­tion of or­ganic mat­ter dur­ing the warm period of the middle Mio­cene. Thus, this feed­back mech­an­ism would be ex­pec­ted to play out dur­ing other warm­ing in­ter­vals in Earth’s his­tory, as well as in any fu­ture warm­ing of the global ocean.

“As we warm up the ocean, it will make it harder for or­ganic car­bon to find its way to be bur­ied in the mar­ine sed­i­ment sys­tem, and that is what we have found in our study – the low­est rates of car­bon se­quest­ra­tion hap­pen when the planet was warm”, said co-au­thor Yige Zhang of Texas A&M Uni­versity. “So that’s not help­ing from this per­spect­ive, in terms of the is­sues that we’re fa­cing in the present day.”

However, the team’s re­search sug­gests that this res­pir­a­tion-like pro­cess pre­vents or­ganic car­bon se­quest­ra­tion from re­du­cing car­bon di­ox­ide emis­sions to the at­mo­sphere. When bac­teria pro­cess the or­ganic car­bon, it is re­turned to its ori­ginal form as CO2.

• Originalpublikation:  Ziye Li, Yi Ge Zhang, Mark Tor­res and Ben­ja­min J. W. Mills: Neo­ge­ne bu­ri­al of or­ga­nic car­bon in the glo­bal oce­an. Na­tu­re 2023. DOI: 10.1038/?s41586-022-05413-6

Source

MARUM – Zen­trum für Mar­ine Um­weltwis­senschaften 2023