Problems with paleoproductivity proxies

Abstract

[1] Difficulties associated with reconstructing past changes in export production were highlighted recently by Averyt and Paytan (2004), who reported substantial disagreement among records developed using different paleoproductivity proxies extracted from two equatorial Pacific piston cores. Proxies included the accumulation rates of barite, excess Ba, and excess Al, as well as elemental ratios of Al/Ti and Ba/Ti. Here we build upon their work by presenting evidence for two factors that contributed to these discrepancies. First, elemental (Ba/Ti and Al/Ti) ratios are influenced by variability in space and time of the flux of Ti to equatorial Pacific sediments, so these proxies cannot be expected to hold a constant relationship to export production. Second, the late Holocene increase in CaCO 3 dissolution has caused concentrations of barite, excess Ba, and excess Al to be enriched in surface sediments relative to the depth interval over which sediment accumulation rates were evaluated in developing the algorithms used by Averyt and Paytan (2004). This produces an error in the accumulation rates of these proxies that varies from core to core, ranging from a few tens of percent to as much as a factor of 3. These errors would have been propagated into the export production algorithms on the basis of fluxes of barite, excess Ba, and excess Al. Furthermore, this bias created by the late Holocene increase in CaCO 3 dissolution will affect the development of any algorithm based on fluxes of sedimentary constituents. These factors must be taken into account in future paleoceanographic reconstructions.

Paleoproductivity and Stratification Across the Subarctic Pacific Over Glacial‐Interglacial Cycles

“…Comparisons of 231 Pa/ 230 Th and other paleoproductivity proxies (e.g., opal fluxes) in the Pacific often yield highly positive correlations (Bradtmiller et al, ; Costa et al, ), but the relationship is stronger in some regions than in others (Dubois et al, ; Lam et al, ; Pichat et al, ). Some degree of interproxy disagreement is not uncommon when reconstructing productivity (e.g., see discussions in Anderson & Winckler, ; Kohfeld & Chase, ; Serno et al, ), and for 231 Pa/ 230 Th it arises because (1) 231 Pa/ 230 Th may be integrating different kinds of productivity (e.g., diatomaceous vs. coccolithophorid) and (2) organic components are susceptible to postdepositional changes in preservation and diagenesis. Opal scavenges 231 Pa/ 230 Th due to both the particle‐flux effect and the particle composition effect, and therefore, opal and 231 Pa/ 230 Th should be highly correlated in regions where surface productivity is dominated specifically by diatoms and other siliceous biota.…”

Mesopelagic carbon remineralization during the European Iron Fertilization Experiment

“…The remineralized carbon in the mesopelagic layer was estimated using an algorithm relating mesopelagic Ba xs contents to the rate of oxygen consumption as deduced from a 1‐D advection‐diffusion‐consumption model applied on highly resolved and precise dissolved O 2 profiles (2m binned data from CTD profiles) along 6°W in the Southern Ocean [ Shopova et al , 1995; Dehairs et al , 1997]: where J O 2 is the O 2 consumption ( μ mol L −1 d −1 ), C respired is the amount of carbon mineralized (in mmol C m −2 d −1 ; further expressed in mg C m −2 d −1 ), Z is the thickness of the water column layer over which mesopelagic Ba xs is calculated, RR is the respiration O 2 :C molar ratio (we used a mean value of 0.733 ± 0.018 for O 2 :C ratios reported in literature [ Broecker and Peng , 1982; Anderson and Winckler , 2005]), meso‐Ba xs is the Ba xs amount that accumulates over the growth season and Ba residual is the background Ba xs signal at zero oxygen consumption, i.e., at zero organic C demand. The residual Ba xs likely depends on the saturation state of the water with respect to barite.…”

Late Miocene carbon isotope records and marine biological productivity: Was there a (dusty) link?

“…Ba/Al ratios in excess of those in regional detrital inputs are often used for qualitative [ Schmitz , 1987] or quantitative reconstructions of the organic carbon export of the seafloor [e.g., Francois et al , 1995; Dymond et al , 1992; Eagle et al , 2003; Reitz et al , 2004]. The approach hinges on assumptions related to the source of Ba being marine biogenic barite, the detrital Ba/Al (or Ba/Ti) background ratio being known and constant through time, and the flux of Al (or Ti if used to normalize instead of Al) being relatively constant through time [ Averyt and Paytan , 2004; Anderson and Winckler , 2005].…”