Late Quaternary Changes in Silicate Utilisation and Upwelling Intensity off Peru - Insights from Silicon and Neodymium Isotopes
The Peruvian coastal upwelling region is characterised by one of the most pronounced oxygen minimum zones in the worlds ocean. The extension and strength of bottom water oxygen depletion has varied in the past as a function of surface water primary productivity and changes in circulation and upwelling intensity. So far, it has proven difficult to disentangle these influences. In this study the stable silicon isotope composition of diatoms (δ30Si) is used to reconstruct surface water silicic acid (Si(OH)4) utilisation during primary productivity. Radiogenic neodymium isotopes (εNd) from Fe-Mn coatings and benthic foraminifers were measured to trace water masses and their mixing, and the detrital fraction was analysed to provide information about sediment provenance, weathering inputs and their transport pathways. Besides the utilisation and water mass mixing effect, other processes such as remineralisation of the diatoms in the water column and in the sediment, influence the signal that is preserved. Here, the first systematic study of the dissolved δ30Si in the water column (δ30SiSi(OH)4) and in diatoms (δ30Siopal) from the underlying sediments in a coastal upwelling region is carried out to shed light onto the importance of these processes. The results show that the δ30SiSi(OH)4 signature strongly depends on upwelling strength, i.e. supply of new nutrients. Surface waters in the realm of intense upwelling are only weakly fractionated with respect to the subsurface source water signatures, whereas surface waters are more strongly fractionated where upwelling is less pronounced due to higher utilisation of the smaller amounts of available Si(OH)4. The δ30Siopal values of picked diatoms in the underlying sediments vary within the order of the expected fractionation between surface waters and diatoms and thus serve as a basis for the reliable use of the δ30Siopal as a proxy for past upwelling conditions. The sediments were also analysed for their εNd and compared to water column measurements from the same area. In general, all phases (seawater, Fe-Mn coatings, benthic foraminifers and detrital material) display a trend from more radiogenic values in the north towards less radiogenic values in the south. This is in agreement with the distribution of the signatures of the Andean hinterland rocks. This is most likely due to extensive exchange between sediments and the water column, which makes it difficult to use the εNd signatures obtained from Fe-Mn coatings or from benthic foraminiferal carbonate as a water mass tracer. Paleo-reconstructions show, that diatom-related productivity along the Peruvian upwelling area has varied greatly during the past 20 kyr. It was characterised by low fluxes of diatom opal during the last glacial and much higher values thereafter, in particular during the early and the latest Holocene. The combined approach of δ30Siopal and the radiogenic isotope composition (Nd, Sr) of Fe-Mn coatings of sediment particles and the detrital material itself reveal that this variability has been related to changes in Si(OH)4 utilisation, mostly controlled by the supply of iron and phosphate released from the shelf sediments via upwelling. During the glacial strong upwelling of subsurface waters with an overall low nutrient content significantly diminished primary productivity. During the early and late Holocene oceanographic conditions along the shelf were alternating between stronger and weaker upwelling. Especially the last 2,500 years were characterised by the largest variability of the system of the past 20,000 years, which coincided with the onset of modern El Nino-Southern Oscillation conditions in the Pacific. The late Holocene was also disrupted by major climatic anomalies, such as the Little Ice Age (LIA). Off Peru, a permanent mean southward migration of the Intertropical Convergence Zone (ITCZ) during the LIA strongly affected the trade wind-driven strength of the upwelling causing permanent El Nino-like conditions. The δ30Siopal show that productivity and nutrient utilisation were weak. The detrital sediment fraction was dominated by local riverine input of lithogenic material due to higher rainfall in the Andean hinterland. At the end of the LIA, the ITCZ shifted northward, which resulted in a pronounced increase in the strength of upwelling off Peru. Especially Si(OH)4 and Fe remineralisation from the shelf sediments was enhanced causing higher nutrient supply and diatom productivity. The radiogenic isotope record of the detrital shelf sediments indicates increased dust transport due to drier conditions under more persistent non-El Nino conditions. Comparison between δ30Siopal and bulk δ15N of organic matter suggests that surface water utilisation has been the predominant factor influencing both signals. The δ15N signature recorded in the sediment was not, as previously assumed, mainly influenced by subsurface water nitrate reduction processes.
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