Large changes in magnetic mineral concentration dependent parameters by more than 1 order of magnitude occur over 50–150 cm intervals in two marine sediment cores from the oxygen minimum zone in the Gulf of Aden. High‐resolution sedimentological and chemical analyses indicate that these intervals are not associated with turbiditic events or sediment reworking, they do not result from changes in carbonate dilution or differences in sediment properties, and they do not correspond to volcanic layers. Magnetic mineralogical analyses reveal a change in magnetic mineral concentration from a magnetite‐goethite assemblage to pure magnetite within the peak. The peaks almost disappear when the abundance of magnetic minerals is calculated after correcting for the magnetic moments of each magnetic mineral. Therefore, under the assumption that the variability of the magnetic parameters results from postdepositional mineralogical transformations, a relatively constant amount of magnetite was present at the surface of the sediment. Changes in redox conditions and nonsteady state diagenesis transformations have effectively been observed along both cores. Large values of total organic carbon coincide with poor preservation of biogenic and detrital magnetite, which reflects reductive dissolution of the finer magnetite grains. At the same levels, Fe 2+ release from reductively dissolved magnetite favored precipitation of goethite. The susceptibility peaks coincide with episodes of magnetite preservation caused by reduced surface productivity and/or enhanced bottom‐water ventilation accompanying northward extension of Glacial Antarctic Intermediate Water into the Indian Ocean.
A multiproxy study of core MD98‐2166 makes it possible to investigate the influence of orbital forcing and sea level changes on Timor Sea sedimentation during the last 260 ka. Spectral analysis reveals a strong imprint of obliquity in all proxies. This is particularly puzzling for the CaCO 3 and total organic carbon (TOC) records since recent data obtained on nearby core MD01‐2378, collected at a shallower water depth, showed a concentration of spectral power in the eccentricity and precession bands. Our results suggest that while sedimentary record in shallower core MD01‐2378 shows a clear low‐latitude response, that of core MD98‐2166 reflects a stronger influence of high‐latitude forcing through deepwater changes. In addition, Rock‐Eval analyses show that part of the organic carbon could be of terrestrial origin, especially during glacial periods. This suggests that glacial/interglacial TOC fluctuations not only reflect changes of marine productivity and/or preservation at the seafloor but also reflect enhanced input of terrestrial material during periods of low sea level.
<p>The Toba volcano super-eruption on the island of Sumatra occurred about 74,000 years ago<sup>[1]</sup>, close to the transition between interglacial Marine Isotope Stage (MIS) 5 and glacial MIS 4. This eruption, called Youngest Toba Tuff (YTT), is currently described as the largest cataclysmic eruption of the Quaternary. However, the impact of this super-eruption on climate is widely debated and its effects on the ocean remains poorly understood.</p><p>The aim of this work is to estimate its impact on oceanic pH at a site near the eruption center. To do so, we measured &#948;<sup>11</sup>B values (pH proxy) on monospecific samples of planktonic foraminifera <em>Globigerinoides ruber</em> and <em>Pulleniatina obliquiloculata</em> from sediment core BAR94-25 (Andaman Sea) using a recently developed method at the Institut de Physique du Globe de Paris (IPGP)<sup>[2]</sup>. <em>G. ruber</em> is a species that thrives preferentially in surface waters, while <em>P. obliquiloculata</em> lives<em> </em>at the thermocline. Therefore, &#948;<sup>11</sup>B measurements on their shells can reconstruct pH variations in surface and thermocline waters, respectively.</p><p>We selected the interval from 258 to 355 cm, corresponding to an age between 57 and 82 ka. This interval contains two clearly visible tephra layers corresponding to the YTT, at the transition from MIS 5 to MIS 4, and to a post-YTT explosive activity during MIS 4. These layers are correlated with a significant decrease in carbonate content (CaCO<sub>3</sub>). Our results indicate a complex pH response during the two concerned volcanic episodes. Thermocline seawater doesn&#8217;t show significant pH decrease during the volcanic episodes compared to the overall signal recorded throughout the studied interval. Conversely, surface seawater shows a much more important pH decrease during part of the volcanic episodes than during the all studied interval. Such decrease in pH during the transition to a glacial state is particularly surprising because an increase in pH, due to the global reduction in atmospheric CO<sub>2</sub>, is rather expected, as shown by previous foraminifera &#948;<sup>11</sup>B&#160;records<sup>[3]</sup>.</p><p>The coupling of CaCO<sub>3</sub> and pH decrease during tephra levels suggests acidification in the Andaman Sea as a consequence of the Toba volcanic eruptive activity. The seawater surface seems much more sensitive to pH changes than the thermocline zone. However, the reduction of carbonate in the two tephra layers may also be due to dilution from ash falling into the sediment. Other analyses, such as measuring the variation of calcification intensity in planktonic foraminifera, are therefore necessary to better interpret these paleo-pH data.</p><p>[1] Storey et al., 2012, <em>PNAS</em>, 109 (46), 18684-18688</p><p>[2] Buisson et al., 2021, <em>JAAS</em>, 36, 2116-2131</p><p>[3] Foster et al., 2008, <em>EPSL</em>, 271, 254-266</p>
