Weaker ocean cir­cu­la­tion led to more car­bon stor­age in the deep sea

As a nat­ural sink for car­bon, the ocean is a cent­ral ele­ment of the Earth’s cli­mate sys­tem. The amount of car­bon re­moved from the sys­tem in the long run de­pends on how much particles con­tain­ing car­bon are stored in the seabed. Here, the avail­ab­il­ity of dis­solved oxy­gen is of cent­ral im­port­ance, as it is con­sumed dur­ing the mi­cro­bial de­com­pos­i­tion of pre­vi­ously formed bio­mass. The dis­tri­bu­tion of oxy­gen in the wa­ter column is primar­ily de­term­ined by the ver­tical cir­cu­la­tion. To an­swer the ques­tion of whether the cor­res­pond­ing con­di­tions in the deep ocean were sub­ject to changes in the re­cent his­tory of the Earth, the au­thors of the new study ex­amined sed­i­ment samples. Chem­ical ele­ments that can be used as in­dic­at­ors for oxy­gen-free con­di­tions and are pre­served in the sed­i­ment over thou­sands to mil­lions of years were ana­lyzed.

Sediment cores from biologically highly productive area analyzed

The sed­i­ment cores avail­able to the team came from the Cape Basin off the west coast of south­ern Africa, from wa­ter depths between 1,000 and 2,500 meters. Due to the ocean cur­rents, the area is one of the most bio­lo­gic­ally pro­duct­ive ones: Cold, nu­tri­ent-rich wa­ter from the depth in­creases the pro­ductiv­ity of phyto­plank­ton. Sink­ing particles of dead or­ganic ma­ter­ial is pro­cessed by mi­croor­gan­isms in the wa­ter column, as well as on the seabed. This pro­cess mostly con­sumes oxy­gen. If large amounts of or­ganic ma­ter­ial sink, this can re­quire more oxy­gen than is sup­plied by the ocean cur­rents. The wa­ter column be­comes “an­oxic”, which means oxy­gen-free.

Oxygen deficiency also detected in the deep-sea during ice age

Us­ing geo­chem­ical sig­na­tures in the sed­i­ments, the re­search­ers were able to prove that much less oxy­gen must have been avail­able in the deep ocean dur­ing the last gla­cial period com­pared to warmer phases. Un­til now, gla­cial peri­ods were known to have a stronger tem­per­at­ure gradi­ent between the poles and the equator that was dir­ectly re­lated to an in­crease in wind cir­cu­la­tion, thus a stronger up­welling of nu­tri­ent-rich wa­ter and, in turn, more in­tens­ive bio­lo­gical pro­duc­tion. It was also known that due to the form­a­tion of po­lar ice caps and the res­ult­ing lower sea level in cold peri­ods, the near-shore up­welling shif­ted to­wards the con­tin­ental slope and thus the deeper parts of the ocean. “What is new about the cur­rent study is that the de­ple­tion of oxy­gen is not lim­ited to wa­ter depths of a few hun­dred to a thou­sand meters but has now also been de­tec­ted at the bot­tom of the ocean”, says co-au­thor Dr. Mat­thias Za­bel from MARUM.

More organic carbon stored at depth

This can es­sen­tially be at­trib­uted to two causes: In­tens­ive de­com­pos­i­tion pro­cesses of the bio­mass that was in­creas­ingly pro­duced dur­ing gla­cial peri­ods con­sumed a lot of oxy­gen. The in­creased con­tent of or­ganic car­bon in the sed­i­ments stud­ied can be seen as a clear in­dic­a­tion that the avail­ab­il­ity of oxy­gen must have been severely re­stric­ted at the same time. “Today, oxy­gen-free zones are found on the shal­low shelf up to a wa­ter depth of a few hun­dred meters, that is at the trans­ition from the con­tin­ental shelf to the open ocean. Dur­ing the Ice Age, on the other hand, the wa­ter of the open ocean was an­oxic at greater depths”, em­phas­izes Dr Florian Scholz. The GEO­MAR biogeo­chem­ist is co-au­thor of the study and head of the Emmy No­ether re­search group ICONOX – Iron cyc­ling in con­tin­ental mar­gin sed­i­ments and the nu­tri­ent and oxy­gen bal­ance of the ocean.

Implications for the global carbon cycle

“From the sed­i­ment samples, we un­der­stand that dur­ing gla­cial peri­ods, or­ganic ma­ter­ial was de­graded less ef­fect­ively in the deep ocean and con­sequently more or­ganic car­bon was bur­ied in the seabed sink”, says Dr Scholz. “By ana­lyz­ing these pro­cesses from Earth's his­tory in more de­tail, we can bet­ter as­sess whether slower cir­cu­la­tion could also lead to in­creased stor­age of hu­man-re­leased car­bon in deep-sea sed­i­ments in the fu­ture”, adds Dr Za­bel, sum­mar­iz­ing the sig­ni­fic­ance of the new study for re­search. “Against the back­ground of the an­thro­po­genic CO₂ in­crease cur­rent cli­mate change, it is cru­cial to de­term­ine and eval­u­ate pro­cesses and mech­an­isms that im­pact the oceanic bot­tom wa­ter oxy­gen con­tent”, the pa­per states.

Original publication:

Na­ta­scha Rie­din­ger, Flo­ri­an Scholz, Mi­chel­le L. Abshire, Mat­thi­as Za­bel: Per­sis­tent deep wa­ter an­oxia in the eas­tern South At­lan­tic du­ring the last ice age. PNAS 2021. DOI: 10.1073/pnas.2107034118