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  • 1
    Publication Date: 2019-01-01
    Print ISSN: 0921-8181
    Electronic ISSN: 1872-6364
    Topics: Geosciences , Physics
    Published by Elsevier
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  • 2
  • 3
    Publication Date: 2015-07-01
    Print ISSN: 2572-4517
    Electronic ISSN: 2572-4525
    Topics: Geosciences
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  • 4
    Publication Date: 2019-01-08
    Description: The response of the Humboldt Current System to future global warming is uncertain. Here we reconstruct alkenone-derived near-surface temperatures from multiple cores along the Peruvian coast to infer the driving mechanisms of upwelling changes for the last 20 kyr. Our records show a deglacial warming consistent with Antarctic ice-core temperatures and a Mid-Holocene cooling, which, in combination with other paleoceanographic records, suggest a strengthening of upwelling conditions. This cooling, during the globally warm Mid-Holocene, is consistent with an intensification of the Walker Circulation and the South Eastern Pacific Subtropical Anticyclone, indicative of La Niña-like conditions in the Tropical Pacific. Surprisingly, oxygen contents in the subsurface increased and productivity was low during the Mid-Holocene, which are at odds with La Niña-like conditions. This suggests that the Humboldt Current System reacts in multiple ways to a warmer world and may even include a reversal in the present day subsurface deoxygenation. ©2018. American Geophysical Union. All Rights Reserved.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 5
    Publication Date: 2013-09-01
    Print ISSN: 0277-3791
    Electronic ISSN: 1873-457X
    Topics: Geography , Geosciences
    Published by Elsevier
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  • 6
  • 7
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    In:  (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 17 [Kumulativ] pp
    Publication Date: 2019-09-23
    Description: The Eastern Tropical South Pacific (ETSP) subsurface waters are affected by one of the strongest and shallowest oxygen minimum zone (OMZ) in the World Ocean, which results from the interplay of high regional primary production (PP) and sluggish subsurface ventilation. The recent expansion of the tropical OMZs suggests that these zones are very vulnerable to anthropogenic climate change, but their sensitivity to changes in PP and in nutrient cycling still remains poorly understood and under debate. We present a core-top compilation of nitrogen isotopes (d15N) measured on a collection of surface sediment retrieved along the Ecuadorian and Peruvian coasts and compare it to the upper 200 m of water column samples to identify which processes may ultimately influence downcore (i.e past) changes in the isotopic signature of bulk nitrogen. We overall find that in O2–depleted subsurface waters, microbial-mediated NO3- loss to N2 leaves a 15N-enriched signal in underlying sediments characteristic of water masses affected by denitrification and/or anammox, so called nitrogen-loss (N-loss) processes. We find that phytoplankton nitrate (NO3-) uptake in surface waters contributes significantly to the sedimentary signal within the high nutrient, low chlorophyll (HNLC) areas of the studied region. We also detect regional differences in the water column and sedimentary signatures of nitrogen isotopes with respect to contrasting oxygenation and PP status. In the North, between 1°N-10°S, subsurface O2 concentrations are too high to allow N-loss processes to take place and account for nitrogen isotopic changes, keeping !15NNO3- values relatively low in the subsurface waters. In contrast, d15NNO3- values increase toward the surface due to partial nitrate utilization in the photic zone. However, an incomplete utilization of NO3- probably triggers d15Nsed values being consistently lower than the !15N of the upwelled NO3-. Further south, between 10 - 15°S within the perennial upwelling region, HNLC conditions are relaxed, resulting in more intense biological production and its associated phytoplankton uptake of surface NO3-. In addition, subsurface O2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox to take place, resulting in elevated subsurface d15NNO3- values in waters feeding the coastal upwelling. Various sedimentary proxies for productivity reflect increasingly higher PP southward, and echoe a parallel north-south d15Nsed gradient reflecting an increasing subsurface N-loss southwards. In addition, the d15N values measured on core-top sediments and on water column samples increase cross-shelf toward the coast, and highlight an intensification of coastal upwelling along the shoreline bringing the oxycline and nitracline closer to the surface. Overall, our mapping effort suggests that the degree of N-loss influencing subsurface d15NNO3- values, the degree of upper ocean stratification, and the degree of near-surface nitrate utilization should all be considered for the interpretation of downcore d15Nsed records in an attempt of reconstructing the spatio-temporal variability of the Peruvian OMZ. Following on this, past changes in the extent of the OMZ were reconstructed on the basis of 4 piston cores selected from regions located outside, at the rim and within the core of the modern OMZ along a latitudinal transect (from 3.5 to 15°S). The !15N values measured on each core cover the entire Holocene and the last deglaciation period. In the northern part along the Ecuadorian margin, the d15N values are low, ranging from 4 to 6‰, with an amplitude of ~1‰ during the Holocene and the last deglaciation period. This is likely due to the absence of OMZ conditions within this area. In contrast, the southern cores show values similar to modern core-tops data and a large amplitude (~4‰) over the past, that we interpret as resulting from variable N-loss processes taking place in the core of the OMZ, from modern time up to the beginning of the last deglaciation. In all the studied cores, we generally observe a strong increase first and then a decreasing trend in the d15N values a along the time period of the deglaciation, followed by relatively stable d15N signatures during the mid-Holocene. Interestingly, low !15N values (~5.5‰) consistently characterize all the studied cores for the mid-Holocene time interval, indicating an overall absence of heavy nitrate in the surface being subsequently uptaken by phytoplankton. As the proxies for export production suggest that there is no straightforward correlation with d15N values during the mid-Holocene, the changes in d15N values are more likely resulting from modifications of subsurface ventilation rather than of nutrient utilization. An increase in upwelling intensity during the mid-Holocene may have allowed for a more efficient ventilation of the upper part of the water column,preventing any N-loss processes to take place, along with injection of NO3- with low d15N within the euphotic zone. A mid-Holocene increase in upwelling intensity, analogously to modern conditions prevailing during austral winters and La Niña events, should also be mirrored in sedimentary records sensitive to regional shifts in the atmospheric circulation such as latitudinal shifts in the Intertropical Convergence Zone (ITCZ). We further investigated how past changes in precipitation along the Ecuadorian margin has evolved over the last 18 000 years. We used two high-resolution XRF records to track downcore changes in the Guayas River runoff. The titanium over calcium ratio is supposed to reveal information on paleo-precipitation shifts along the western flanks of the equatorial Andes. When compared to other deglacial to Holocene rainfall records located across the tropical South American continent, different modes of variability become apparent. The records of rainfall variability imply that changes in the hydrological cycle at orbital and sub-orbital timescales were different from western to eastern South America. Orbital forcing caused an antiphase behavior in rainfall trends between eastern and western equatorial South America. In contrast, millennial-scale rainfall changes, remotely connected to the North Atlantic climate variability, led to homogenously wetter conditions over eastern and western equatorial South America during North Atlantic cold spells. These results may provide helpful diagnostics for testing the regional rainfall sensitivity in climate models and help to refine rainfall projections in South America for the next century, and corroborate the mid-Holocene increase in upwelling activity which was likely responsible for pushing the ITCZ north of the Ecuadorian margin at this time.
    Type: Thesis , NonPeerReviewed
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  • 8
    Publication Date: 2019-09-23
    Description: We present the first systematic study of the silicon isotope composition in the water column (δ30SiSi(OH)4) and in diatoms (δ30Sidiatom) from the underlying surface sediments in a coastal upwelling region. The surface waters upwelling on the shelf off Peru are mainly fed by southward flowing subsurface waters along the coast, which show a mean δ30SiSi(OH)4 of +1.5‰. The concentration of dissolved silicic acid (Si(OH)4) increases towards the south in these waters and with increasing water depth, suggesting lateral mixing with water masses from the south and intense remineralisation of particulate biogenic silica (bSiO2) in the water column and in the surface sediments. Surface waters in the realm of the most intense upwelling between 5°S and 15°S have only marginally elevated δ30SiSi(OH)4 values (δ30SiSi(OH)4 = +1.7‰) with respect to the source Si isotope composition, whereas further north and south, where upwelling is less pronounced, surface waters are more strongly fractionated (δ30SiSi(OH)4 up to +2.8‰) due to the stronger utilisation of the smaller amounts of available Si(OH)4. The degree of Si(OH)4 utilisation in the surface waters along the shelf estimated from the Si(OH)4 concentration data ranges from 51% to 93%. The δ30Sidiatom values of hand-picked diatoms in the underlying surface sediments vary from +0.6‰ to +2.0‰, which is within the range of the expected fractionation between surface waters and diatoms. The fractionation signal in the surface waters produced during formation of the diatoms is reflected by the δ30Sidiatom values in the underlying sediments, with the lowest δ30Sidiatom values in the main upwelling region. The silicon isotope compositions of bSiO2 (δ30SibSiO2) from the same surface sediment samples are generally much lower than the δ30Sidiatom signatures indicating a significant contamination of the bSiO2 with biogenic siliceous material other than diatoms, such as sponge spicules. This shift towards lighter δ30SibSiO2 values by up to −1.3‰ compared to δ30Sidiatom signatures for the same surface sediment samples potentially biases the interpretation of δ30Si paleorecords from sediments with low bSiO2 concentrations, and thus the reconstruction of past Si(OH)4 utilisation in surface waters.
    Type: Article , PeerReviewed
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  • 9
    Publication Date: 2019-09-23
    Description: We present a high-resolution marine record of sediment input from the Guayas River, Ecuador, that reflects changes in precipitation along western equatorial South America during the last 18ka. We use log (Ti/Ca) derived from X-ray Fluorescence (XRF) to document terrigenous input from riverine runoff that integrates rainfall from the Guayas River catchment. We find that rainfall-induced riverine runoff has increased during the Holocene and decreased during the last deglaciation. Superimposed on those long-term trends, we find that rainfall was probably slightly increased during the Younger Dryas, while the Heinrich event 1 was marked by an extreme load of terrigenous input, probably reflecting one of the wettest period over the time interval studied. When we compare our results to other Deglacial to Holocene rainfall records located across the tropical South American continent, different modes of variability become apparent. The records of rainfall variability imply that changes in the hydrological cycle at orbital and sub-orbital timescales were different from western to eastern South America. Orbital forcing caused an antiphase behavior in rainfall trends between eastern and western equatorial South America. In contrast, millennial-scale rainfall changes, remotely connected to the North Atlantic climate variability, led to homogenously wetter conditions over eastern and western equatorial South America during North Atlantic cold spells. These results may provide helpful diagnostics for testing the regional rainfall sensitivity in climate models and help to refine rainfall projections in South America for the next century.
    Type: Article , PeerReviewed
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  • 10
    Publication Date: 2019-09-23
    Type: Conference or Workshop Item , NonPeerReviewed
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