Benthic Cham­ber Mod­ule

In the highly het­ero­gen­eous ocean eco­sys­tem, the sed­i­ment- bot­tom wa­ter in­ter­face rep­res­ents a bound­ary zone that con­nects the two oceanic realms - the sed­i­ment and the wa­ter column. At the sea­floor the sed­i­ment-wa­ter in­ter­face is one of the most im­port­ant trans­ition zones for solute ex­change, both in time and space. The benthic cham­ber mod­ule en­ables in situ meas­ure­ments of the total ex­change rates (e.g oxy­gen, meth­ane, nu­tri­ents, DIC) between the sed­i­ment and the wa­ter column. This meas­ure­ment in­teg­rates all rel­ev­ant solute trans­port pro­cesses (dif­fu­sion, ad­vec­tion and fauna-me­di­ated trans­port).

The instrument
The benthic cham­ber mod­ule con­sists of a sup­port frame that car­ries a ti­tanium cyl­in­der, a plexi­glass in­cub­a­tion cham­ber and a syr­inge sample/​in­jec­tion device. The ti­tanium cyl­in­der houses all elec­tron­ics, a data ac­quis­i­tion com­puter, syr­inge sampler con­trol­ler and two sensor-amp­li­fi­ers. The meas­ure­ment prin­ciple is based on in situ in­cub­a­tion of a cer­tain volume of sed­i­ment along with well-mixed bot­tom wa­ter, dur­ing which the change of con­cen­tra­tions of solutes over time is re­cor­ded. The cent­rally stirred cham­ber has a dia­meter of 19 cm and hence en­closes an area of 284 cm2 to­gether with 10 - 20 cm high (equi­val­ent to 4-6 l) overly­ing bot­tom wa­ter. Con­tinu­ous meas­ure­ments of the con­cen­tra­tion of oxy­gen and sulf­ide as well as pH are real­ized with sensors moun­ted on the lid of the in­cub­a­tion cham­ber. The con­cen­tra­tion of other com­pon­ents, like dis­solved meth­ane, nu­tri­ents and DIC is assed via re­trieval of dis­crete samples at pre-pro­grammed sampling in­ter­vals from the en­closed wa­ter body. These samples and sensor meas­ure­ments ac­quired dur­ing the in­cub­a­tion period al­low as­sess­ing the total fluxes and trans­port rates across the sed­i­ment sur­face. The benthic cham­ber mod­ule is op­er­ated by ROVs, which also po­s­i­tion the mod­ule at the tar­geted meas­ure­ment sites. A one-way valve on the lid of the in­cub­a­tion cham­ber re­leases the ex­cess of wa­ter, caused by the in­ser­tion of the cham­ber in the sed­i­ment, en­sur­ing a gentle place­ment and start of the meas­ure­ments.