Energy, commons and the rest

Thoughts on energy as a commons, science and other stuffs by Cecile Blanchet

NW American margin


Sedimentation, Climate and Paleoceanography on the NW American Margin

Climatic and oceanic variability of the last glacial/interglacial cycle (120,000 yrs BP): magnetic and geochemical properties of sediments from subtropical Northwestern American margin.
Ph-D Thesis supervised by Nicolas Thouveny and Laurence Vidal, and defended in September 2006 (Aix en Provence, France).

The magnetic and geochemical properties of four marine sediment cores from the north-western American margin allowed reconstructing the dynamics of terrigenous sedimentation and post-depositional processes, which were related to climatic and oceanological changes during the last glacial-interglacial cycle (0-120,000 yrs). The cores have been retrieved from three sites located on the Californian and Mexican coasts: the Santa Barbara Basin (35°N), the margin of Baja California (23°N) and the Gulf of Tehuantepec (15°N). Deposition of terrigenous and biogenic particles follow a seasonal pattern and sediments accumulate at rates of 35 to 150 cm/ka. Organic matter degradation in the water column and weak ventilation of the water masses lead to low dissolved oxygen contents in the water masses between 300 and 800 m depth, which is denoted as the Eastern Pacific oxygen minimum zone. The magnetic parameters are used to trace the concentration, nature or grain sizes changes of the magnetic particles. Based on their magnetic signature, we could decipher between terrigenous iron oxides (magnetite, hematite, goethite) and authigenic iron sulfides (greigite and pyrrhotite). The relative contents of major and trace elements, which were measured using a X-ray fluorescence (XRF) scanner for several key intervals in various cores, help to strengthen the rock magnetic interpretations. Oxygen and carbon stable isotope ratios were measured for planktonic and benthic foraminifera that were picked in the core from Gulf of Tehuantepec. The isotope ratios allowed reconstructing changes in physical and chemical characteristics of the water column, thereby providing clues to past water mass distribution. The sedimentary sequences, which were dated by correlation of magnetic susceptibility profiles, calibrated 14C ages and identification of paleomagnetic excursions, cover the glacial-interglacial transition (0-40 ka) and one of them covers the last climatic cycle (0-120 ka). High concentration in iron oxides during glacial intervals for all sites suggests strong terrigenous inputs on the NW American margin at these times, and particularly strong aeolian inputs during the last glacial maximum (20-26 ka BP). Occurrence of various types of solid iron sulfides in the sediments of the Santa Barbara Basin was tentatively related to different levels of bottom waters oxygenation (but this interpretation was modified later, see Blanchet et al., 2009a). In the Gulf of Tehuantepec, the thermocline was less deep during the deglaciation (16-10 ka BP) than during the Holocene (10-0 ka BP) and the presence of authigenic gypsum (CaSO4) in the sediments, which is probably formed during reoxidation of pyrite, suggests a better oxygenation of the bottom waters during the glacial periods (interpretation also modified, see Blanchet et al, 2009b).

Example of land-ocean interactions on the NW American Margin: Southward Trade Wind blast over the Tehuantepec Ithmus, which induces cooler sea surface temperatures (blue colors in the upper panel) and plankton blooming (whitish colors in the lower panel). Such winds are also known to transport dust to the ocean

C. L. Blanchet, N. Thouveny, L. Vidal, G. Leduc, K. Tachikawa, E. Bard, L. Beaufort (2007). Terrigenous input response to glacial/interglacial climatic variations over southern Baja California: a rock magnetic approach. Quaternary Science Letters (doi:10.1016/j.quascirev.2007.07.008)

Data: Magnetic data and XRF data

The sediments deposited off south-western Baja California have recorded millennial-scale fluctuations in biogenic compounds and trace metals accumulation during the last glacial period [Ortiz, J.D., O’Connell, S.B., DelViscio, J., Dean, W.E., Carriquiry, J.D., Marchitto, T., Zheng, Y., van Geen, A., 2004. Enhanced marine productivity off western North America during warm climate intervals of the past 52 ky. Geology 32, 521–524; Dean, W.E., Zheng, Y., Ortiz, J.D., van Geen, A., 2006. Sediment Cd and Mo accumulation in the oxygen-minimum zone off western Baja California linked to global climate over the past 52 ka. Paleoceanography 21, PA4209]. Since the variations in trace metals concentration appear to result from dilution with nonbiogenic matter, the variability of the terrigenous sedimentation needs to be addressed. Therefore, we performed rock magnetic and geochemical analysis on a 38 m-long sediment core collected from the slope off Baja California. The temporal framework provided by 14C dating and identification of palaeoclimatic transitions allows assigning the sequence to the last glacial–interglacial cycle (last 120 ka). The comparison of magnetic and geochemical properties led to retain the hypothesis of a primary modulation of iron oxides quantity and quality by terrigenous input variations, with a secondary diagenetic amplification. Two magnetic mineral input regimes are distinguished: (i) magnetic susceptibility variations reveal changes in titano-magnetite concentration related to fluvial transport of the terrigenous fraction; (ii) coercivity changes allow detecting variations of hematite or goethite concentrations, minerals generally issued from aeolian deflation of weathered rock surfaces. These two regimes are paced by two distinct climatic forcing: the millennial-scale changes in titano-magnetite input are related to the northern hemisphere climatic variability whilst the record of wind-blown magnetic mineral input contains its major power in the precessional frequency band, with higher input during low insolation periods (and conversely). This record highlights the great sensibility of this region to high and low latitudes climatic regimes.

Figure 10 from Blanchet et al., 2007, which shows changes in magnetite (b) and hematite or goethite (c) contents during the past 120 thousands of years in sediments from the Baja California Margin. This indicate that both northern and tropical climatic forcing (here depicted by the oxygen isotopes in the GRIP ice core (a) and the summer insolation at 23°N (d)) control the terrigenous inputs at this location.

C. L. Blanchet, N. Thouveny, and L. Vidal (2009). Formation and preservation of greigite (Fe3S4) in sediments from the Santa Barbara Basin: Implications for paleoenvironmental changes during the past 35 ka. Paleoceanographyi (doi: 10.1029/2008PA001719, 2009)

Diagenetic processes are known to modify sedimentary records, but they can also reveal important paleoenvironmental changes. Here we investigate variations in sedimentary iron diagenesis and depositional environments for the last 35 ka by analyzing the rock magnetic and geochemical properties of two sediment cores collected in the Santa Barbara Basin (California). In organic-rich sediments, early diagenesis often leads to partial dissolution of detrital iron oxides and stepwise formation of authigenic pyrite (FeS2). The pyritisation process takes place following several geochemical pathways, sometimes involving intermediate iron sulphide species such as greigite (Fe3S4). Sedimentary conditions in the basin appear to have recurrently favoured preservation of greigite (identified by its magnetic properties) and inhibited its complete transformation into pyrite. The Holocene interval contains a series of centimeter-thick greigite-bearing layers that are associated with large flood deposits, which are known in the basin as ‘‘gray layers.’’ We propose that greigite preservation was enabled in these sediments by their relative enrichment in reactive iron over organic matter and/or hydrogen sulfide (because of their high ratio of terrigenous over organic material), which limited pyritization reactions. Within the glacial deposits, formation and preservation of meter-thick greigite layers occurred in terrigenous-rich and organic-poor sedimentary layers and is proposed to result from a similar diagenetic process to that in the Holocene greigite-bearing layers (dominance of reactive iron over organic matter and/or HS). The terrigenous enrichments in the glacial greigite-bearing layers are probably related to climatic or sea level changes because they occur at times of massive iceberg releases in the North Atlantic, the so-called Heinrich events.

Figure 8 from Blanchet et al., 2009. Magnetic properties of sediments helped identify greigite enrichements (on the pictures high SIRM/K values, underlined in grey). This authigenic sulfur formed during events of large terrigenous inputs: 1) associated with large flood deposits during the Holocene (not shown) and 2) occurring during Heinrich Events during the glacial interval (figure).

C. Blanchet, S. Kasten, L. Vidal, S. Poulton, R. Ganeshram and N. Thouveny (2012). Influence of diagenesis on the stable isotopic composition of biogenic carbonates from the Gulf of Tehuantepec oxygen minimum zone. Geochemistry, Geophysics, Geosystems (doi: 10.1029/2011GC003800)

Data: Foram isotopes and Inorganic calcite isotopes

In order to evaluate the influence of diagenetic and post-sampling processes on the stable oxygen and carbon isotope compositions of biogenic carbonates, we conducted a multiproxy study of organic-rich sediments from the eastern Pacific oxygen minimum zone. Core MD02-2520, which was retrieved from the Gulf of Tehuantepec (Mexico), has seasonal laminations and covers the last 40 kyr. Together with the presence of gypsum crystals and inorganic calcite aggregates, the occurrence of large excursions in the stable oxygen and carbon isotope records of both planktonic and benthic foraminifera (as large as +3‰ in d18O and 5‰ in d13C) point to significant secondary transformations. Storage-related gypsum precipitation was ruled out since it implies sulfide reoxidation by oxygen that triggers biogenic calcite dissolution, which proved to be of minor importance here. Instead, precipitation of authigenic calcite during early diagenesis appears to be the most likely process responsible for the observed isotopic excursions. The d13C composition for inorganic calcite aggregates (5 to 7‰) suggests a major contribution from anaerobic oxidation of organic matter. The d34S composition for gypsum crystals (10 to +15‰) suggests a major contribution from anaerobic reoxidation of authigenic sulfides, potentially involving reactions with metal oxides and sulfur disproportionation. A minor part of the gypsum might possibly have formed as a result of local pore water salinity increases induced by gas hydrate formation.

Scanning Electron Microscope Image of a specimen of Globigerinoides Ruber, with very well preserved calcitic spines (although this foram was picked from a sample full of gypsum crystals…)
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