Search for: All records

Carbon isotope records from alkenone biomarkers (ε_p37:2) produced by haptophyte algae are frequently used for atmospheric CO₂paleobarometry, but this method has yielded inconsistent results during periods where CO₂variations are known independently. Recent syntheses of algal cultures have quantitatively demonstrated that ε_p37:2indeed records CO₂information: ε_p37:2increases as aqueous CO₂concentrations increase relative to carbon demand. However, interpretations of ε_p37:2are complicated by irradiance, where higher irradiance yields higher ε_p37:2. Here we examine the roles of physiology and environment in setting ε_p37:2in the ocean. We compile water‐column and sediment core‐top ε_p37:2data and add new core‐top measurements, including estimates of cell sizes and growth rates of the alkenone‐producing population. In support of culture studies, we find irradiance to be a key control on ε_p37:2in the modern ocean. We test a culture‐derived model of ε_p37:2and find that the quantitative relationships calibrated in culture experiments can be used to predict ε_p37:2in sediment samples. In water‐column samples, the model substantially overestimates ε_p37:2, largely resulting from higher irradiance at the depth of sample collection than the integrated light conditions under which the sampled biomass was produced and vertically mixed to the collection depth. We argue that the theory underpinning the conventional diffusive alkenone carbon isotope fractionation model, including the “b” parameter, is not supported by field data and should not be used to reconstruct past CO₂changes. Future estimates of CO₂from ε_p37:2should use empirical or mechanistic models to quantitatively account for irradiance and cell size variations.

In tropical and sub‐tropical mixed siliciclastic–carbonate depositional systems, fluvial input andin situneritic carbonate interact over space and time. Despite being the subject of many studies, controls on partitioning of mixed sediments remains controversial. Mixed sedimentary records, from Ashmore Trough shelf edge and slopes (southern Gulf of Papua), are coupled with global sea‐level curves and anchored to Marine Isotope Stage stratigraphy to constrain models of sediment accumulation at two different timescales for the past 130 kyr: (i) 100 kyr scale for last glacial cycle; and (ii) millennial scale for last deglaciation. During the last glacial cycle, carbonate production and accumulation were primarily controlled by sea‐level fluctuations. Export of neritic carbonate to the slopes was initiated during re‐flooding of previously exposed reefs and continued during Marine Isotope Stage 5e and 1 interglacial sea‐level highs. Siliciclastic fluxes to the slope were controlled by interplay of sea level, shelf physiography and oceanic currents. Heterogeneous accumulation of siliciclastic mud on the slope, took place during Marine Isotope Stage 5d to Marine Isotope Stage 3 sea‐level fall. Siliciclastics reached adjacent depocentres during Marine Isotope Stage 2. Coralgal reef and oolitic–skeletal sand resumed at the shelf edge during the subsequent stepwise sea‐level rise of the last deglaciation. Contemporaneous, abrupt siliciclastic input from increased precipitation and fluvial discharge illustrates that climate controlled deglacial sedimentation. Siliciclastic input persisted untilca8.5 ka. Carbonate accumulation waned at the shelf edge afterca14 ka, whereas it increased on the slopes sinceca11.5 ka, when previously exposed reef and bank tops were re‐flooded. When comparing the last sea‐level cycle sedimentation patterns of the southern Gulf of Papua with other coeval mixed systems, sea level and shelf physiography emerge as primary controls on deposition at the 100 kyr scale. At the millennial scale, siliciclastic input was also controlled by climate change during the unstable atmospheric and oceanic conditions of the last deglaciation.