Patagonian ash on sub‐Antarctic South Georgia: expanding the tephrostratigraphy of southern South America into the Atlantic sector of the Southern Ocean

Abstract

Fingerprinting non‐visible volcanic ash (cryptotephra) enables precise dating and time synchronization of palaeoclimate archives. Recent analytical advances allow us to strengthen and expand existing tephrostratigraphical frameworks and harness the full potential of this powerful geochronological tool. Here, we present geochemical (electron microprobe) and chronological (14C) evidence to show that ash found in a peat section on the sub‐Antarctic island of South Georgia correlates to the ∼2950 cal a BP Alpehué eruption of the Chilean Sollipulli volcano. This discovery marks the first tephra fingerprint on South Georgia and expands the recently refined tephrostratigraphy of Patagonia ∼3000 km into the Southern Ocean. As deposition of this tephra horizon coincides with a major shift in regional atmospheric conditions, future finds may yield information on the spatiotemporal pattern of Southern Ocean climate dynamics.

Rapid Tephra Identification in Geological Archives With Computed Tomography: Experimental Results and Natural Applications

“…For example, offsets exist: this is particularly noticeable toward the lower part of the analyzed section of core DB2. As shown in Figure 6, the shard maximum of the So-A horizon presented by Oppedal et al (2018) sits ∼2.5 cm higher up than its likely expression in our scan (115 cm). We argue that this disparity stems from a combination of 1) differences in sampling resolution-while manual samples were extracted at 1 cm intervals, DB2 was scanned at ∼20 µm resolution, 2) disturbance imparted during storage or (U-channel) sampling-Figure 6 shows the presence of cracks that were not present when glass shards were extracted, and 3) layer tilting-close investigation of the volume rendering of the So-A horizon at 115 cm reveals a ∼18°angle.…”

“…However, because of computational and analytical limitations (CT Scanning section), we could not achieve smaller (<20 µm) voxel sizes for this study. Instead, we opted for a different approach by spiking one of our analyzed natural sediment archives, core DB-2 from South Georgia (Oppedal et al, 2018; Conventionally Analyzed Natural Archives and Application on Natural Sediment Archives sections), with a cryptic (∼1,000 shard) CI horizon near the core top (Figure 6). In this way, partial volume effects impact the CT grayscale values of tephra standards and horizons in equal measure, so that the former can still be used to help pinpoint the density range of volcanic ash.…”

“…As we could not obtain valuable ice core material, this effort was focused on peat and minerogenic cores. We scanned the 95-120 cm section of peat core DB-2 from sub-Antarctic South Georgia (54°13′S, 36°38′W), which was previously investigated at 1 cm intervals by Oppedal et al (2018). This interval contains a distinct 304 shards per cubic centimetre rhyolitic (avg.…”

“…As revealed in Figure 5A, this basic statistic distinguishes all cryptic horizons from background variability. Owing to the greater density difference between ash (∼2.3-2.5 g/cm 3 ) and silty clay host FIGURE 6 | CT scans (2-D orthoslices and 3-D representations) and 1 cm resolution manual shard (>20 μm) counts of the analyzed 95-120 cm section of peat core DB-2 with red bars (Oppedal, van der Bilt, Balascio et al, 2018; Conventionally Analyzed Natural Archives section). The histogram highlights the grayscale value distribution of rhyolitic glass (red) and heavier clastic material (orange) dispersed throughout the core (including partial volume effects).…”

Evidence for distal transport of reworked Andean tephra: Extending the cryptotephra framework from the Austral volcanic zone

“…The Falkland Islands (Islas Malvinas) are well placed to extend the tephrostratigraphy of southern South America as they lie beneath the central jet of the SWW (50-55⁰S), and downwind of the Andean Southern Volcanic Zone (SVZ; 33-46 °S) and the Austral Andean Volcanic Zone (AVZ; 49-55°S). Since the last glacial period 74 volcanic centres in these zones are known to have been active (Fontijn et al, 2014(Fontijn et al, , 2016, and cryptotephra deposits in Isla de los Estados (Unkel et al, 2008), the Falkland Islands (Hall et al, 2001), South Georgia (Oppedal et al, 2018) and the Antarctic ice cores (Kurbatov et al, 2006;Narcisi et al, 2012) have been linked with Andean volcanoes. The Falkland Islands are therefore ideally placed to study palaeo-ash clouds extending from the Southern Andes during the Lateglacial-Holocene transition.…”