We diagnose the relative influences of local-scale biogeochemical cycling and regional-scale ocean circulation on Atlantic barium cycling by analysing four new depth profiles of dissolved Ba concentrations and isotope compositions from the South and tropical North Atlantic. These new profiles exhibit systematic vertical, zonal and meridional variations that reflect the influence of both local-scale barite cycling and large-scale ocean circulation. Epipelagic decoupling of dissolved Ba and Si reported previously in the tropics is also found to be associated with significant Ba isotope heterogeneity. As such, we contend that this decoupling originates from the depth segregation of opal and barite formation but is exacerbated by weak vertical mixing. Zonal influence from isotopically-'heavy' water masses in the western North Atlantic evidence the advective inflow of Ba-depleted Upper Labrador Sea Water, which is not seen in the eastern basin or the South Atlantic. Meridional variations in Atlantic Ba isotope systematics below 2000 m appear entirely controlled by conservative mixing. Using an inverse isotopic mixing model, we calculate the Ba isotope composition of the Ba-poor northern end-member as +0.45 ‰ and the Ba-rich southern end-member +0.26 ‰, relative to NIST SRM 3104a. The near-conservative behaviour of Ba below 2000 m indicates that Ba isotopes can serve as an independent tracer of the provenance of northern-versus southern-sourced water masses in the deep Atlantic Ocean. This finding may prove useful in palaeoceanographic studies, should appropriate sedimentary archives be identified, and offers new insights into the processes that cycle Ba in seawater.

Geochemical analyses of sedimentary barites (barium sulfates) in the geological record have yielded fundamental insights into the chemistry of the Archean environment and evolutionary origin of microbial metabolisms. However, the question of how barites were able to precipitate from a contemporary ocean that contained only trace amounts of sulfate remains controversial. Here we report dissolved and particulate multi-element and barium-isotopic data from Lake Superior that evidence pelagic barite precipitation at micromolar ambient sulfate. These pelagic barites likely precipitate within particle-associated microenvironments supplied with additional barium and sulfate ions derived from heterotrophic remineralization of organic matter. If active during the Archean, pelagic precipitation and subsequent sedimentation may account for the genesis of enigmatic barite deposits. Indeed, barium-isotopic analyses of barites from the Paleoarchean Dresser Formation are consistent with a pelagic mechanism of precipitation, which altogether offers a new paradigm for interpreting the temporal occurrence of barites in the geological record.

18The concentrations, distributions, and stable carbon isotopes (δ 13 C) of plant waxes carried 19 by fluvial suspended sediments contain valuable information about terrestrial ecosystem 20 characteristics. To properly interpret past changes recorded in sedimentary archives it is crucial 21 to understand the sources and variability of exported plant waxes in modern systems on seasonal 22 to inter-annual timescales. To determine such variability, we present concentrations and δ 13 C 23 compositions of three compound classes (n-alkanes, n-alcohols, n-alkanoic acids) in a 34-month 24 time series of suspended sediments from the outflow of the Congo River. Carbon-normalized concentrations and relative abundances of n-alcohols (19 -58% of 37 total plant-wax lipids) and n-alkanoic acids (26 -76%) respond rapidly to seasonal changes in 38 runoff, indicating that they are mostly derived from a recently entrained local source. In contrast, 39 a lack of correlation with discharge and low, stable relative abundances (5 -16%) indicate that 40 n-alkanes better represent a catchment-integrated signal with minimal response to discharge 41 seasonality. Comparison to published data on other large watersheds indicates that this 42 phenomenon is not limited to the Congo River, and that analysis of multiple plant-wax lipid 43 classes and chain lengths can be used to better resolve local vs. distal ecosystem structure in river 44 catchments. 45 3

Barium isotopes in mid-ocean ridge basalts reveal that the global upper mantle is contaminated with small amounts of sediment.