Veränderungen im Silizium- und Stickstoff-Kreislauf vor Peru während der vergangenen 20.000 Jahre

The Peruvian coastal upwelling region is characterized by very high surface water primary productivity. Enhanced oxygen consumption during the degradation of organic matter and sluggish ventilation lead to suboxic to anoxic conditions in subsurface waters resulting in one of the largest Oxygen Minimum Zones (OMZs) globally, which causes enhanced N-loss processes in the water column and shelf sediments. The strength and spatial extent of these processes in the past have been regulated by changes in oceanic and atmospheric circulation, which have in turn been controlled by global and regional climatic changes. Reconstructions of surface water dissolved silicic acid (Si(OH)4) and nitrate (NO3-) utilization as a function of primary productivity and the strength of pelagic denitrification in the oxygen-depleted subsurface waters supplying the upwelling have shown to be recorded by stable silicon isotope composition (δ30Si) of diatom and bulk sediment nitrogen isotope compositions. However, species-specific Si isotope enrichment factors for some marine diatoms species have shown to potentially affect past δ30Si records of diatoms due to past shifts in the dominance of particular diatom species. To account for potential biases due to the prevalence of different diatom species off Peru surface sediment δ30Si of small mixed (11-32µm; δ30SibSi) and of large handpicked (Coscinodiscus; >150µm; δ30Sicoscino) diatom species were investigated and compared to the diatom assemblages.Das peruanische Auftriebsgebiet ist gekennzeichnet durch eine sehr hohe Primärproduktivität im Oberflächenwasser. Die Zersetzung von organischem Material und eine relativ langsame Wassermaschenmischung führen zu starker Sauerstoffnutzung und starken sub-oxischen bis an-oxischen Bedingungen und eine der global größten Sauerstoffminimumzonen (OMZs), in der es zum Abbau von Nitrat in der Wassersäule und in Schelfsedimenten kommt. Die Stärke und das Ausmaß dieser Prozesse in der Vergangenheit sind direkt abhängig von der ozeanischen und atmosphärischen Zirkulation, die durch globale und regionale Klimaveränderungen beeinflusst wurden. Zur Rekonstruktion der Oberflächenwassernutzung von der gelöster Kieselsäure (Si(OH)4) und des Nitrats (NO3-) während der Primärproduktivität, sowie der Intensität von der Denitrifizierung in der Wassersäule haben sich die Verhältnisse von stabilen Silizium-Isotopen in Diatomeen (δ30Si) und von sedimentären Stickstoff Isotopen (δ15Nbulk) als Hilfsmittel bewährt. Allerdings, haben artenabhängige Si-Isotopen Fraktionierungsfaktoren verschiedener Diatomeenarten gezeigt, dass Veränderungen in der Dominanz der Arten einen starken Einfluss auf die Aufzeichnung vergangener δ30Si- Schwankungen haben können. Um die Auswirkungen der veränderlichen Häufigkeit verschiedener Diatomeenarten auf die δ30Si-Signaturen in den Sedimentarchiven des peruanischen Schelfs zu bestimmen, wurden hier deshalb zum Einen δ30Si Werte von kleinen gemischten Diatomeenarten (11-32µm; δ30SibSi) und zum anderen von großen handgepickten Diatomeen (Coscinodiscus; >150µm) aus den Oberflächensedimenten untersucht und mit der Diatomeen Vergesellschaftung verglichen

Changes in silicon and nitrogen cycling off Peru during the past 20,000 years - Veränderungen im Silizium- und Stickstoff-Kreislauf vor Peru während der vergangenen 20.000 Jahre

The Peruvian coastal upwelling region is characterized by very high surface water primary productivity. Enhanced oxygen consumption during the degradation of organic matter and sluggish ventilation lead to suboxic to anoxic conditions in subsurface waters resulting in one of the largest Oxygen Minimum Zones (OMZs) globally, which causes enhanced N-loss processes in the water column and shelf sediments. The strength and spatial extent of these processes in the past have been regulated by changes in oceanic and atmospheric circulation, which have in turn been controlled by global and regional climatic changes. Reconstructions of surface water dissolved silicic acid (Si(OH)4) and nitrate (NO3-) utilization as a function of primary productivity and the strength of pelagic denitrification in the oxygen-depleted subsurface waters supplying the upwelling have shown to be recorded by stable silicon isotope composition (δ30Si) of diatom and bulk sediment nitrogen isotope compositions. However, species-specific Si isotope enrichment factors for some marine diatoms species have shown to potentially affect past δ30Si records of diatoms due to past shifts in the dominance of particular diatom species. To account for potential biases due to the prevalence of different diatom species off Peru surface sediment δ30Si of small mixed (11-32µm; δ30SibSi) and of large handpicked (Coscinodiscus; >150µm; δ30Sicoscino) diatom species were investigated and compared to the diatom assemblages

Changes in diatom productivity and upwelling intensity off Peru since the Last Glacial Maximum: Response to basin-scale atmospheric and oceanic forcing

New records of stable silicon isotope signatures (δ30Si) together with concentrations of biogenic opal and organic carbon from the central (9° S) and northern (5° S) Peruvian margin reveal changes in diatom productivity and nutrient utilization during the past 20,000 years. The findings are based on a new approach using the difference between the δ30Si signatures of small (11-32μm) and large (>150μm) diatom fractions (Δ30Sicoscino-bSi) in combination with the variance in diatom assemblages for reconstruction of past upwelling intensity. Combination of our records with two previously published records from the southern upwelling area off Peru (12-15° S) shows a general decoupling of the environmental conditions at the central and southern shelf mainly caused by a northward shift of the main upwelling cell from its modern position (12-15° S) towards 9° S during Termination 1. At this time only moderate upwelling intensity and productivity levels prevailed between 9° S and 12° S interpreted by a more northerly position of Southern Westerly Winds and the South Pacific Subtropical High. Furthermore, a marked decrease in productivity at 12-15° S during Heinrich Stadial 1 coincided with enhanced biogenic opal production in the Eastern Equatorial Pacific, which was induced by a southward shift of the Intertropical Convergence zone and enhanced northeasterly trade winds. Modern conditions were only established at the onset of the Holocene. Past changes in preformed δ30Si signatures of subsurface waters reaching the Peruvian Upwelling System did not significantly affect the preserved δ30Si signatures

Controls on the Silicon Isotope Composition of Diatoms in the Peruvian Upwelling

The upwelling area off Peru is characterized by exceptionally high rates of primary productivity, mainly dominated by diatoms, which require dissolved silicic acid (dSi) to construct their frustules. The silicon isotope compositions of dissolved silicic acid (δ 30 Si dSi ) and biogenic silica (δ 30 Si bSi ) in the ocean carry information about dSi utilization, dissolution, and water mass mixing. Diatoms are preserved in the underlying sediments and can serve as archives for past nutrient conditions. However, the factors influencing the Si isotope fractionation between diatoms and seawater are not fully understood. More δ 30 Si bSi data in today’s ocean are required to validate and improve the understanding of paleo records. Here, we present the first δ 30 Si bSi data (together with δ 30 Si dSi ) from the water column in the Peruvian Upwelling region. Samples were taken under strong upwelling conditions and the bSi collected from seawater consisted of more than 98% diatoms. The δ 30 Si dSi signatures in the surface waters were higher (+1.7‰ to +3.0‰) than δ 30 Si bSi (+1.0‰ to +2‰) with offsets between diatoms and seawater (Δ 30 Si) ranging from −0.4‰ to −1.0‰. In contrast, δ 30 Si dSi and δ 30 Si bSi signatures were similar in the subsurface waters of the oxygen minimum zone (OMZ) as a consequence of a decrease in δ 30 Si dSi . A strong relationship between δ 30 Si bSi and [dSi] in surface water samples supports that dSi utilization of the available pool (70 and 98%) is the main driver controlling δ 30 Si bSi . A comparison of δ 30 Si bSi samples from the water column and from underlying core-top sediments (δ 30 Si bSi_ sed. ) in the central upwelling region off Peru (10°S and 15°S) showed good agreement (δ 30 Si bSi_ sed. = +0.9‰ to +1.7‰), although we observed small differences in δ 30 Si bSi depending on the diatom size fraction and diatom assemblage. A detailed analysis of the diatom assemblages highlights apparent variability in fractionation among taxa that has to be taken into account when using δ 30 Si bSi data as a paleo proxy for the reconstruction of dSi utilization in the region

Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis

Benthic fluxes of dissolved silica (Si) from sediments into the water column are driven by the dissolution of biogenic silica (bSiO2) and terrigenous Si minerals and modulated by the precipitation of authigenic Si phases. Each of these processes has a specific effect on the isotopic composition of silica dissolved in sediment pore waters such that the determination of pore water δ30Si values can help to decipher the complex Si cycle in surface sediments. In this study, the δ30Si signatures of pore fluids and bSiO2 in the Guaymas Basin (Gulf of California) were analyzed, which is characterized by high bSiO2 accumulation and hydrothermal activity. The δ30Si signatures were investigated in the deep basin, in the vicinity of a hydrothermal vent field, and at an anoxic site located within the pronounced oxygen minimum zone (OMZ). The pore fluid δ30Sipf signatures differ significantly depending on the ambient conditions. Within the basin, δ30Sipf is essentially uniform averaging +1.2 ± 0.1 ‰ (1SD). Pore fluid δ30Sipf values from within the OMZ are significantly lower (0.0 ± 0.5 ‰, 1SD), while pore fluids close to the hydrothermal vent field are higher (+2.0 ± 0.2 ‰, 1SD). Reactive transport modelling results show that the δ30Sipf is mainly controlled by silica dissolution (bSiO2 and terrigenous phases) and Si precipitation (authigenic aluminosilicates). Precipitation processes cause a shift to high pore fluid δ30Sipf signatures, most pronounced at the hydrothermal site. Within the OMZ however, additional dissolution of isotopically depleted Si minerals (e.g. clays) facilitated by high mass accumulation rates of terrigenous material (MARterr) is required to promote the low δ30Sipf signatures while precipitation of authigenic aluminosilicates seems to be hampered by high water / rock ratios. Guaymas OMZ δ30Sipf values are markedly different from those of the Peruvian OMZ, the only other marine setting where Si isotopes have been investigated to constrain early diagenetic processes. These differences highlight the fact that δ30Sipf signals in OMZs worldwide are not alike and each setting can result in a range of δ30Sipf values as a function of the environmental conditions. We conclude that the benthic silica cycle is more complex than previously thought and that additional Si isotope studies are needed to decipher the controls on Si turnover in marine sediment and the role of sediments in the marine silica cycle

Nutrient and Silicon Isotope Dynamics in the Laptev Sea and Implications for Nutrient Availability in the Transpolar Drift

Realistic prediction of the near-future response of Arctic Ocean primary productivity to ongoing warming and sea ice loss requires a mechanistic understanding of the processes controlling nutrient bioavailability. To evaluate continental nutrient inputs, biological utilization and the influence of mixing and winter processes in the Laptev Sea, the major source region of the Transpolar Drift, we compare observed with preformed concentrations of dissolved inorganic nitrogen (DIN), phosphorus (DIP), silicic acid (DSi) and silicon isotope compositions of DSi (δ30SiDSi) obtained for two summers (2013, 2014) and one winter (2012). In summer, preformed nutrient concentrations persisted in the surface layer of the southeastern Laptev Sea, while diatom-dominated utilization caused intense northward drawdown and a pronounced shift in δ30SiDSi from +0.91 to +3.82 ‰. The modeled Si isotope fractionation suggests that DSi in the northern Laptev Sea originated from the Lena River during the spring freshet, while in the southeastern Laptev Sea it was continuously supplied by it during the summer. Primary productivity fueled by river-borne nutrients was enhanced by admixture of DIN- and DIP-rich Atlantic-sourced waters to the surface, either by convective mixing during the previous winter or by occasional storm-induced stratification breakdowns in late summer. Substantial enrichments of DSi (+240 %) and DIP (+90 %) beneath the Lena River plume were caused by sea ice-driven redistribution and remineralization. Predicted weaker stratification on the outer Laptev shelf will enhance DSi utilization and removal through greater vertical DIN supply, which will limit DSi export and reduce diatom-dominated primary productivity in the Transpolar Drift. Key Points - Surface DIN, DIP, DSi and Si isotope dynamics are controlled by marine and riverine inputs and uptake by phytoplankton - Strong DIP and DSi enrichments beneath the Lena River plume are due to sea ice-driven nutrient redistribution and remineralization - Enhanced DSi utilization in the Laptev Sea will lead to a reduced diatom-dominated primary productivity in the Transpolar Drif

Dissolved silicon isotope dynamics in large river estuaries

Estuarine systems are of key importance for the riverine input of silicon (Si) to the ocean, which is a limiting factor of diatom productivity in coastal areas. This study presents a field dataset of surface dissolved Si isotopic compositions (30SiSi(OH)4) obtained in the estuaries of three of the world’s largest rivers, the Amazon (ARE), Yangtze (YRE), and Pearl (PRE), which cover different climate zones. While 30SiSi(OH)4 behaved conservatively in the YRE and PRE supporting a dominant control by water mass mixing, significantly increased 30SiSi(OH)4 signatures due to diatom utilization of Si(OH)4 were observed in the ARE and reflected a Si isotopic enrichment factor 30 of −1.0±0.4‰ (Rayleigh model) or −1.6±0.4‰ (steady state model). In addition, seasonal variability of Si isotope behavior in the YRE was observed by comparison to previous work and most likely resulted from changes in water residence time, temperature, and light level. Based on the 30 value obtained for the ARE, we estimate that the global average 30SiSi(OH)4 entering the ocean is 0.2-0.3‰ higher than that of the rivers due to Si retention in estuaries. This systematic modification of riverine Si isotopic compositions during estuarine mixing, as well as the seasonality of Si isotope dynamics in single estuaries, needs to be taken into account for better constraining the role of large river estuaries in the oceanic Si cycle

Sea surface and subsurface circulation dynamics off equatorial Peru during the last ~17 kyr

The complex deglacial to Holocene oceanographic development in the Gulf of Guayaquil (Eastern Equatorial Pacific) is reconstructed for sea surface and subsurface ocean levels from (isotope) geochemical proxies based on marine sediment cores. At sea surface, southern sourced Cold Coastal Water and tropical Equatorial Surface Water/Tropical Surface Water are intimately related. In particular since ~10 ka, independent sea surface temperature proxies capturing different seasons emphasize the growing seasonal contrast in the Gulf of Guayaquil, which is in contrast to ocean areas further offshore. Cold Coastal Water became rapidly present in the Gulf of Guayaquil during the austral winter season in line with the strengthening of the Southeast Trades, while coastal upwelling off Peru gradually intensified and expanded northward in response to a seasonally changing atmospheric circulation pattern affecting the core locations intensively since 4 ka BP. Equatorial Surface Water, instead, was displaced and Tropical Surface Water moved northward together with the Equatorial Front. At subsurface, the presence of Equatorial Under Current-sourced Equatorial Subsurface Water was continuously growing, prominently since ~10–8 ka B.P. During Heinrich Stadial 1 and large parts of the Bølling/Allerød, and similarly during short Holocene time intervals at ~5.1–4 ka B.P. and ~1.5–0.5 ka B.P., the admixture of Equatorial Subsurface Water was reduced in response to both short-term weakening of Equatorial Under Current strength from the northwest and emplacement by tropical Equatorial Surface Water, considerably warming the uppermost ocean layers

Factors controlling plankton community production, export flux, and particulate matter stoichiometry in the coastal upwelling system off Peru

Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The production of organic material is fueled by upwelling of nutrient-rich deep waters and high incident light at the sea surface. However, biotic and abiotic factors can modify surface production and related biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predictions of their future functioning requires understanding of the mechanisms driving the biogeochemical cycles therein. In this field experiment, we used in situ mesocosms as tools to improve our mechanistic understanding of processes controlling organic matter cycling in the coastal Peruvian upwelling system. Eight mesocosms, each with a volume of ∼55 m3, were deployed for 50 d ∼6 km off Callao (12∘ S) during austral summer 2017, coinciding with a coastal El Niño phase. After mesocosm deployment, we collected subsurface waters at two different locations in the regional oxygen minimum zone (OMZ) and injected these into four mesocosms (mixing ratio ≈1.5 : 1 mesocosm: OMZ water). The focus of this paper is on temporal developments of organic matter production, export, and stoichiometry in the individual mesocosms. The mesocosm phytoplankton communities were initially dominated by diatoms but shifted towards a pronounced dominance of the mixotrophic dinoflagellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift coincided with a short-term increase in production during the A. sanguinea bloom, which left a pronounced imprint on organic matter C : N : P stoichiometry. However, C, N, and P export fluxes did not increase because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, export fluxes during the study were decoupled from surface production and sustained by the remaining plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant for most of the experiment. We explain this constancy by light limitation through self-shading by phytoplankton and by inorganic nitrogen limitation which constrained phytoplankton growth. Thus, gain and loss processes remained balanced and there were few opportunities for blooms, which represents an event where the system becomes unbalanced. Overall, our mesocosm study revealed some key links between ecological and biogeochemical processes for one of the most economically important regions in the oceans