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1. Expedition 355 Preliminary Report: Arabian Sea Monsoon

   https://doi.org/10.14379/iodp.pr.355.2015  

   Pandey, D.K. ; Clift, P.D. ; Kulhanek, D.K. ; Ando, S. ; Bendle, J.A.P. ; Bratenkov, S. ; Griffith, E.M. ; Gurumurthy, G.P. ; Hahn, A. ; Iwai, M. ; et al ( July 2015 , Preliminary report)

   null (Ed.)

   The Indian (southwest) summer monsoon is one of the most intense climatic phenomena on Earth. Its long-term development has been linked to the growth of high topography in South and Central Asia. The Indian continental margin, adjoining the Arabian Sea, offers a unique opportunity to investigate tectonic–climatic interactions and the net impact of these processes on weathering and erosion of the western Himalaya. During International Ocean Discovery Program Expedition 355, two sites (U1456 and U1457) were drilled in Laxmi Basin in the eastern Arabian Sea to document the coevolution of mountain building, weathering, erosion, and climate over a range of timescales. In addition, recovering basement from the eastern Arabian Sea provides constraints on the early rifting history of the western continental margin of India with special emphasis on continental breakup between India and the Seychelles and its relationship to the plume-related volcanism of the Deccan Plateau. Drilling and coring operations during Expedition 355 recovered sediment from Sites U1456 and U1457 in the Laxmi Basin, penetrating 1109.4 and 1108.6 m below seafloor (mbsf), respectively. Drilling reached sediment dated to 13.5–17.7 Ma (late early to early middle Miocene) at Site U1456, although with a large hiatus between the lowermost sediment and overlying deposits dated to <10.9 Ma. At Site U1457, a much longer hiatus occurs near the base of the cored section, spanning from 10.9 to ~62 Ma. At both sites, hiatuses span ~8.2–9.2 and ~3.6–5.6 Ma, with a possible condensed section spanning ~2.0–2.6 Ma, although the total duration for each hiatus is slightly different between the two sites. A major submarine fan draining the western Himalaya and Karakoram must have been supplying sediment to the eastern Arabian Sea since at least ~17 Ma. Sand mineral assemblages indicate that the Greater Himalayan Crystalline Sequence was fully exposed to the surface by this time. Most of the recovered sediment appears to be derived from the Indus River and includes minerals that are unique to the Indus Suture Zone, in particular glaucophane and hypersthene, most likely originating from the structural base of the Kohistan arc. Pliocene sandy intervals at Site U1456 were deposited in lower fan “sheet lobe” settings, with intervals of basin plain turbidites separated by hemipelagic muddy sections deposited during the Miocene. Site U1457 is more distal in facies, reflecting its more marginal setting. No major active lobe appears to have affected the Laxmi Basin since the Middle Pleistocene (~1.2 Ma). We succeeded in recovering sections spanning the 8 Ma climatic transition, when monsoon intensity is believed to have changed strongly, although the nature of this change awaits postcruise analysis. We also recovered sediment from a large mass transport deposit measuring ~330 and ~190 m thick at Sites U1456 and U1457, respectively. This section includes an upper sequence of slump-folded muddy and silty rocks, as well as underlying calcarenites and limestone breccias, together with smaller amounts of volcanic clasts, all of which are likely derived from the western Indian continental shelf. Identification of similar facies on the regional seismic lines in Laxmi Basin suggests that these deposits form parts of one of the world’s largest mass transport deposits. Coring of igneous basement was successful at Site U1457. Recovery of massive basalt and associated volcaniclastic sediment at this site should address the key questions related to rifting and volcanism associated with formation of Laxmi Basin. Geochemical analysis is required to understand the petrogenesis and thus the tectonic setting of volcanism that will reveal whether it is oceanic basalt or volcanic rock contaminated by underlying continental crust or continental flood basalt. However, the fact that the lavas are massive and have few vesicles implies water depths of eruption likely deeper than 2000 m. This precludes opening of the basin in the presence of a major mantle thermal anomaly, such as that associated with the Deccan Large Igneous Province. Other observations made at the two sites during Expedition 355 provide vital constraints on the rift history of this margin. Heat flow measurements at the two drill sites were calculated to be ~57 and ~60 mW/m2. Such heat flow values are compatible with those observed in average oceanic crust of 63–84 Ma age, as well as with the presence of highly extended continental crust. Postcruise analyses of the more than ~1722 m of core will provide further information about the nature of tectonic–climatic interactions in this global type area for such studies. 

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2. The first day of the Cenozoic

   https://doi.org/10.1073/pnas.1909479116  

   Gulick, Sean P. ; Bralower, Timothy J. ; Ormö, Jens ; Hall, Brendon ; Grice, Kliti ; Schaefer, Bettina ; Lyons, Shelby ; Freeman, Katherine H. ; Morgan, Joanna V. ; Artemieva, Natalia ; et al ( September 2019 , Proceedings of the National Academy of Sciences)

   Highly expanded Cretaceous–Paleogene (K-Pg) boundary section from the Chicxulub peak ring, recovered by International Ocean Discovery Program (IODP)–International Continental Scientific Drilling Program (ICDP) Expedition 364, provides an unprecedented window into the immediate aftermath of the impact. Site M0077 includes ∼130 m of impact melt rock and suevite deposited the first day of the Cenozoic covered by <1 m of micrite-rich carbonate deposited over subsequent weeks to years. We present an interpreted series of events based on analyses of these drill cores. Within minutes of the impact, centrally uplifted basement rock collapsed outward to form a peak ring capped in melt rock. Within tens of minutes, the peak ring was covered in ∼40 m of brecciated impact melt rock and coarse-grained suevite, including clasts possibly generated by melt–water interactions during ocean resurge. Within an hour, resurge crested the peak ring, depositing a 10-m-thick layer of suevite with increased particle roundness and sorting. Within hours, the full resurge deposit formed through settling and seiches, resulting in an 80-m-thick fining-upward, sorted suevite in the flooded crater. Within a day, the reflected rim-wave tsunami reached the crater, depositing a cross-bedded sand-to-fine gravel layer enriched in polycyclic aromatic hydrocarbons overlain by charcoal fragments. Generation of a deep crater open to the ocean allowed rapid flooding and sediment accumulation rates among the highest known in the geologic record. The high-resolution section provides insight into the impact environmental effects, including charcoal as evidence for impact-induced wildfires and a paucity of sulfur-rich evaporites from the target supporting rapid global cooling and darkness as extinction mechanisms. 

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3. Formation of the crater suevite sequence from the Chicxulub peak ring: A petrographic, geochemical, and sedimentological characterization

   https://doi.org/10.1130/B36020.1  

   Kaskes, Pim ; de Graaff, Sietze J. ; Feignon, Jean-Guillaume ; Déhais, Thomas ; Goderis, Steven ; Ferrière, Ludovic ; Koeberl, Christian ; Smit, Jan ; Wittmann, Axel ; Gulick, Sean P.S. ; et al ( July 2021 , GSA Bulletin)

   null (Ed.)

   This study presents a new classification of a ∼100-m-thick crater suevite sequence in the recent International Ocean Discovery Program (IODP)-International Continental Scientific Drilling Program (ICDP) Expedition 364 Hole M0077A drill core to better understand the formation of suevite on top of the Chicxulub peak ring. We provide an extensive data set for this succession that consists of whole-rock major and trace element compositional data (n = 212) and petrographic data supported by digital image analysis. The suevite sequence is subdivided into three units that are distinct in their petrography, geochemistry, and sedimentology, from base to top: the ∼5.6-m-thick non-graded suevite unit, the ∼89-m-thick graded suevite unit, and the ∼3.5-m-thick bedded suevite unit. All of these suevite units have isolated Cretaceous planktic foraminifera within their clastic groundmass, which suggests that marine processes were responsible for the deposition of the entire M0077A suevite sequence. The most likely scenario describes that the first ocean water that reached the northern peak ring region entered through a N-NE gap in the Chicxulub outer rim. We estimate that this ocean water arrived at Site M0077 within 30 minutes after the impact and was relatively poor in rock debris. This water caused intense quench fragmentation when it interacted with the underlying hot impact melt rock, and this resulted in the emplacement of the ∼5.6-m-thick hyaloclastite-like, non-graded suevite unit. In the following hours, the impact structure was flooded by an ocean resurge rich in rock debris, which caused the phreatomagmatic processes to stop and the ∼89-m-thick graded suevite unit to be deposited. We interpret that after the energy of the resurge slowly dissipated, oscillating seiche waves took over the sedimentary regime and formed the ∼3.5-m-thick bedded suevite unit. The final stages of the formation of the impactite sequence (estimated to be <20 years after impact) were dominated by resuspension and slow atmospheric settling, including the final deposition of Chicxulub impactor debris. Cumulatively, the Site M0077 suevite sequence from the Chicxulub impact site preserved a high-resolution record that provides an unprecedented window for unravelling the dynamics and timing of proximal marine cratering processes in the direct aftermath of a large impact event. 

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4. The Temporal and Environmental Context of Early Animal Evolution: Considering All the Ingredients of an “Explosion”

   https://doi.org/https://doi.org/10.1093/icb/icy088  

   Sperling, E.A. ; Stockey, R.G. ( January 2018 , Integrative and comparative biology)

   Animals originated and evolved during a unique time in Earth history—the Neoproterozoic Era. This paper aims to discuss (1) when landmark events in early animal evolution occurred, and (2) the environmental context of these evolutionary milestones, and how such factors may have affected ecosystems and body plans. With respect to timing, molecular clock studies—utilizing a diversity of methodologies—agree that animal multicellularity had arisen by ∼800 million years ago (Ma) (Tonian period), the bilaterian body plan by ∼650 Ma (Cryogenian), and divergences between sister phyla occurred ∼560–540 Ma (late Ediacaran). Most purported Tonian and Cryogenian animal body fossils are unlikely to be correctly identified, but independent support for the presence of pre-Ediacaran animals is recorded by organic geochemical biomarkers produced by demosponges. This view of animal origins contrasts with data from the fossil record, and the taphonomic question of why animals were not preserved (if present) remains unresolved. Neoproterozoic environments demanding small, thin, body plans, and lower abundance/rarity in populations may have played a role. Considering environmental conditions, geochemical data suggest that animals evolved in a relatively low-oxygen ocean. Here, we present new analyses of sedimentary total organic carbon contents in shales suggesting that the Neoproterozoic ocean may also have had lower primary productivity—or at least lower quantities of organic carbon reaching the seafloor—compared with the Phanerozoic. Indeed, recent modeling efforts suggest that low primary productivity is an expected corollary of a low-O2 world. Combined with an inability to inhabit productive regions in a low-O2 ocean, earliest animal communities would likely have been more food limited than generally appreciated, impacting both ecosystem structure and organismal behavior. In light of this, we propose the “fire triangle” metaphor for environmental influences on early animal evolution. Moving toward consideration of all environmental aspects of the Cambrian radiation (fuel, heat, and oxidant) will ultimately lead to a more holistic view of the event. 

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5. Keratose sponges in ancient carbonates – A problem of interpretation

   https://doi.org/10.1111/sed.13059  

   Neuweiler, Fritz ; Kershaw, Stephen ; Boulvain, Frédéric ; Matysik, Michał ; Sendino, Consuelo ; McMenamin, Mark ; Munnecke, Axel ( April 2023 , Sedimentology)

   Reports of diverse vermiform and peloidal structures in Neoproterozoic to Mesozoic open marine to peritidal carbonates include cases interpreted to be keratose sponges. However, living keratose sponges have elaborate, highly elastic skeletons of spongin (a mesoscopic end‐member of a hierarchical assemblage of collagenous structures) lacking spicules, thus have poor preservation potential in contrast to the more easily fossilized spicule‐bearing sponges. Such interpreted fossil keratose sponges comprise diverse layered, network, amalgamated, granular and variegated microfabrics of narrow curved, branching, vesicular–cellular to irregular areas of calcite cement, thought to represent former spongin, embedded in microcrystalline to peloidal carbonate. Interpreted keratose sponges are presented in publications almost entirely in two‐dimensional (thin section) studies, usually displayed normal to bedding, lacking mesoscopic three‐dimensional views in support of a sponge body fossil. For these structures to be keratose sponges critically requires conversion of the spongin skeleton into the calcite cement component, under shallow‐burial conditions and this must have occurred prior to compaction. However, there is no robust petrographic–geochemical evidence that the fine‐grained carbonate component originated from sponge mummification (automicritic body fossilsviacalcification of structural tissue components) because in the majority of cases the fine‐grained component is homogenous and thus likely to be deposited sediment. Thus, despite numerous studies, verification of fossil keratose sponges is lacking. Although some may be sponges, all can be otherwise explained. Alternatives include: (i) meiofaunal activity; (ii) layered microbial (spongiostromate) accretion; (iii) sedimentary peloidal to clotted micrites; (iv) fluid escape and capture resulting in bird's eye to vuggy porosities; and (v) moulds of siliceous sponge spicules. Uncertainty of keratose sponge identification is fundamental and far‐reaching for understanding: (i) microfacies and diagenesis where they occur; (ii) fossil assemblages; and (iii) wider aspects of origins of animal clades, sponge ecology, evolution and the systemic recovery from mass extinctions. Thus, alternative explanations must be considered.

    

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