Latitudinal changes in elemental composition of smectite in deep sea sediments and its implication for microbial activity along the transect of equatorial Pacific Ocean

In order to determine the sediment provenance and potential iron (Fe) source in abyssal environment, deep sea surface sediments from a transect of the equatorial Pacific Ocean (0–10°N along ~131°W) were investigated for clay mineral assemblages, the elemental composition of smectite, and biogeochemical characteristics, including total organic carbon (TOC), total nitrogen (TN), and adenosine triphosphate (ATP), all of which are closely related to the specific sediment source and cellular process. Not only the relative composition of clay minerals but also two types of smectite (Fe-rich and Al-rich) were detected along the latitudinal transect. The composition of smectite collected close to the equator (0–6°N) was more Fe-rich compared to that of smectite collected at 7–10°N, which was Al-rich. These two distinct compositional groups indicate the existence of at least two sources of smectite at the study sites: (1) Fe-rich smectite supplied from South America and (2) Al-rich smectite derived from Asia and North America. These results suggest that the elemental composition of smectite can be used alongside other conventional proxies to differentiate the provenances of abyssal sediments in the central Pacific Ocean based on the southernmost latitude with Asian and North American sources (7°N) as the present-day mean Intertropical Convergence Zone (ITCZ) position. Moreover, an increase in the Fe content of smectite (N0: 22.1 wt%, N6: 19.3 wt%, and N10: 16.5 wt%) is positively correlated with the latitudinal distribution of ATP concentrations in surface sediments. Therefore, Fe-rich smectite may be important as a Fe source associated with microbial activity in deep sea environments with potential consequences for microbe–mineral interactions.