The polygenetic hypothesis of Yedoma origin – comparing grain-size distributions of Alaskan and Siberian Yedoma

The formation of late Pleistocene Yedoma in western Beringia (Siberia) is still widely debated. Moreover, different geological and cryostratigraphical views on Yedoma exist between researchers focusing on western or eastern Beringia (Alaska and Northwest Canada). These differences largely concern the prominence of the role of eolian processes. In particular, previous studies on Yedoma in the Yukon Territories and Alaska interpret these deposits as being largely loess or re-transported loess (muck). In contrast, several hypotheses have emerged over decades of research in the extensive Siberian Yedoma region, including (1) alluvial genesis, (2) ice-sheet-dammed basin sediments, (3) deltaic formation, (4) proluvial and slope deposits, (5) cryogenic-aeolian deposits, (6) nival deposits, and (7) polygenetic origins. Characteristics that most studies agree on include the dominance of large syngenetic ice wedges, mainly allochthonous silty to sandy sediment deposition in low-center polygons in combination with deposition of mainly autochthonous organic remnants from plants and animals, very harsh continental, glacial climate conditions. In terms of landscape and relief characteristics, various Yedoma types seem to exist across the extensive region where Yedoma does occur, ranging from spatially confined Yedoma valley fills including slopes to vast accumulation plains on Arctic lowlands and shelves. Accordingly, we here support the notion that Yedoma may have different depositional properties and genetic origins under a common frame of similar environmental and climatic conditions during the Late Pleistocene. This hypothesis is known as polygenetic formation of Yedoma. An important aspect of Yedoma is the dominating presence of excess ground ice. Ice wedges and segregated intra-sedimentary ice constitute the majority of this deposit by volume (50-80%) in most Yedoma regions and are thus one of the most critical factors in deposit genesis in contrast to accumulations of silty to sandy deposits in temperate regions. The Yedoma Ice Complex formation includes cryogenic processes such as cryogenic weathering, ice segregation, syngenetic ice wedge formation and growth, secondary sediment deformation and reworking due to ground ice, and cryosol formation (including phases dominated by orthels, turbels, or histels). All these processes were promoted by long-lasting harsh continental climate conditions. Furthermore, the formation of large polygon ice-wedge nets and thick continuous sequences of frozen deposits is closely related to the persistence of stable, poorly drained, low topographic gradient accumulation areas. A comprehensive cryolithogenic concept of polygenetic Yedoma formation combines cryogenic weathering, periglacial material transport and accumulation, and relief shaping under cold-arid climate conditions and considers two general formation processes: (1) the primary accumulation in low-centered ice-wedge polygons and (2) the syngenetic freezing and ice-wedge growth in non-glaciated Arctic lowlands under cold-arid climate of the late Pleistocene. Following this concept Yedoma represents a specific periglacial facies whose formation is controlled by the interaction of several climate, landscape and geological preconditions typical for non-glaciated Arctic and sub-Arctic lowlands and foothills. In contrast to the pure aeolian (loess) or glacial hypotheses, the proposed cryolithogenic concept integrates several previous formation concepts and in particular takes the important role of ground ice in the deposit formation process into account. Generally, this corresponds to the polygenetic character of Yedoma formation. It also includes the potential for several sediment sources, weathering processes, and pathways by which sediments in typical periglacial landscapes can build up the Ice Complex horizon. Generally, Yedoma consists of often poorly sorted sediments with maxima in the silt and fine sand, but also coarse sand and gravels can be included. Grain-size characteristics differ with study sites and within horizons (Schirrmeister et al. 2011). To understand the local characteristics as well as the regional variation in the sedimentology of the late Pleistocene Yedoma deposits, we analyzed the grain-size distribution (GSD) of hundreds of samples from dozens of Yedoma sites: The multi-modal GSD of two examples from the Bykovsky Peninsula (Siberia) and the Colville River (Alaska) already indicate a variety of sediment production, transport and depositional processes (Fig. 1). To disentangle these processes a robust end-member modeling analysis (EMMA) was performed on Yedoma sediments of the two sites following Dietze et al. (2012) and Dietze et al. (2014). Multiple robust grain-size end-members (rEM) were unmixed (Fig. 1). The average robust model explains 85.6 % of the total grain-size variability in Colville and only 53.5 % of the Bykovsky Yedoma grain-size variability, the latter being indicative for very poor sorting and high heterogeneity of the Bykovsky sediments in contrast to Colville sediments. Both sites were composed of five robust end-members with modes at 4, 17, 36, 210 and 340 m that explain 25, 28, 36, 8.2 and 2.8 % of the variance of Colville sediments. The Bykovsky rEM had modes at 5, 27, 120, 210 and 310 m comparable to Colville rEMs, each of them explaining 8.9, 23, 22, 21 and 25 % of the grain-size fractions that can be explained by EMMA. The various grain-size end-members supports the hypothesis of polygenetic Yedoma origin involving multiple transport and depositional processes. Although both sites were dominated by silt-transporting and depositing processes, an important amount of finer fractions were deposited at Colville, whereas rather high amounts of coarse- to fine-sandy deposits composed the Bykovsky Yedoma. Developing a site-specific interpretation of past depositional processes helps understanding the formation conditions of thaw susceptible Yedoma deposits in the terrestrial Arctic and could be crucial for understanding the future trajectories of this unique kind of permafrost in a warming Arctic

The genesis of Yedoma Ice Complex permafrost – grain-size endmember modeling analysis from Siberia and Alaska

The late Pleistocene Yedoma Ice Complex is an ice-rich and organic-bearing type of permafrost deposit widely distributed across Beringia and is assumed to be especially prone to deep degradation with warming temperature, which is a potential tipping point of the climate system. To better understand Yedoma formation, its local characteristics, and its regional sedimentological composition, we compiled the grain-size distributions (GSDs) of 771 samples from 23 Yedoma locations across the Arctic; samples from sites located close together were pooled to form 17 study sites. In addition, we studied 160 samples from three non-Yedoma ice-wedge polygon and floodplain sites for the comparison of Yedoma samples with Holocene depositional environments. The multimodal GSDs indicate that a variety of sediment production, transport, and depositional processes were involved in Yedoma formation. To disentangle these processes, a robust endmember modeling analysis (rEMMA) was performed. Nine robust grain-size endmembers (rEMs) characterize Yedoma deposits across Beringia. The study sites of Yedoma deposits were finally classified using cluster analysis. The resulting four clusters consisted of two to five sites that are distributed randomly across northeastern Siberia and Alaska, suggesting that the differences are associated with rather local conditions. In contrast to prior studies suggesting a largely aeolian contribution to Yedoma sedimentation, the wide range of rEMs indicates that aeolian sedimentation processes cannot explain the entire variability found in GSDs of Yedoma deposits. Instead, Yedoma sedimentation is controlled by local conditions such as source rocks and weathering processes, nearby paleotopography, and diverse sediment transport processes. Our findings support the hypothesis of a polygenetic Yedoma origin involving alluvial, fluvial, and niveo-aeolian transport; accumulation in ponding waters; and in situ frost weathering as well as postdepositional processes of solifluction, cryoturbation, and pedogenesis. The characteristic rEM composition of the Yedoma clusters will help to improve how grain-size-dependent parameters in permafrost models and soil carbon budgets are considered. Our results show the characteristic properties of ice-rich Yedoma deposits in the terrestrial Arctic. Characterizing and quantifying site-specific past depositional processes is crucial for elucidating and understanding the trajectories of this unique kind of ice-rich permafrost in a warmer future

Immature and Maturation-Resistant Human Dendritic Cells Generated from Bone Marrow Require Two Stimulations to Induce T Cell Anergy In Vitro

Immature dendritic cells (DC) represent potential clinical tools for tolerogenic cellular immunotherapy in both transplantation and autoimmunity. A major drawback in vivo is their potential to mature during infections or inflammation, which would convert their tolerogenicity into immunogenicity. The generation of immature DC from human bone marrow (BM) by low doses of GM-CSF (lowGM) in the absence of IL-4 under GMP conditions create DC resistant to maturation, detected by surface marker expression and primary stimulation by allogeneic T cells. This resistence could not be observed for BM-derived DC generated with high doses of GM-CSF plus IL-4 (highGM/4), although both DC types induced primary allogeneic T cell anergy in vitro. The estabishment of the anergic state requires two subsequent stimulations by immature DC. Anergy induction was more profound with lowGM-DC due to their maturation resistance. Together, we show the generation of immature, maturation-resistant lowGM-DC for potential clinical use in transplant rejection and propose a two-step-model of T cell anergy induction by immature DC

Limits to reproduction and seed size-number trade-offs that shape forest dominance and future recovery

International audienceThe relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacity to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundation for assessing fitness in forest trees. Four major findings emerged. First, seed production is not constrained by a strict trade-off between seed size and numbers. Instead, seed numbers vary over ten orders of magnitude, with species that invest in large seeds producing more seeds than expected from the 1:1 trade-off. Second, gymnosperms have lower seed production than angiosperms, potentially due to their extra investments in protective woody cones. Third, nutrient-demanding species, indicated by high foliar phosphorus concentrations, have low seed production. Finally, sensitivity of individual species to soil fertility varies widely, limiting the response of community seed production to fertility gradients. In combination, these findings can inform models of forest response that need to incorporate reproductive potential

Limits to reproduction and seed size-number tradeoffs that shape forest dominance and future recovery

The relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacity to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundation for assessing fitness in forest trees. Four major findings emerged. First, seed production is not constrained by a strict trade-off between seed size and numbers. Instead, seed numbers vary over ten orders of magnitude, with species that invest in large seeds producing more seeds than expected from the 1:1 trade-off. Second, gymnosperms have lower seed production than angiosperms, potentially due to their extra investments in protective woody cones. Third, nutrient-demanding species, indicated by high foliar phosphorus concentrations, have low seed production. Finally, sensitivity of individual species to soil fertility varies widely, limiting the response of community seed production to fertility gradients. In combination, these findings can inform models of forest response that need to incorporate reproductive potential