ALBERT

Description: The Red Sea is an important example of a continental rift transitioning slowly to an oceanic basin. However, structures that can inform us of how that transition occurred have been poorly reported because deep seismic reflection data capable of imaging basement under the rift sediments are generally lacking publicly. Three lines of multichannel seismic reflection data have recently been published revealing structures on the Nubian side of the central part of the basin. In this study, we reassess these data in the light of recent studies of the central Red Sea. Over continental crust, the data reveal reflection sequences likely due to strata at or near the base of the evaporites, in two cases with varied dips suggesting the presence of syn-rift growth stratigraphy. Almost all of those reflections dip downwards towards the rift axis, not away as would be expected from tilted fault blocks of bookshelf faulting types. That observation, and low relief of basement, confirm inferences made earlier based on gravity anomalies that this part of the Red Sea lacks large-relief fault escarpments and is most likely a syn-rift sag basin. In the transition to oceanic crust, an abnormally broad magnetic anomaly of estimated Chron 5 age is found not to be associated with structures such as sills, so it likely arises from deeper sources. One of the seismic lines traverses a ridge in Bouguer gravity anomalies that runs across the axis. This feature has previously been interpreted as a volcanic ridge similar to those observed at other ultra-slow spreading ridges. The seismic data reveal diffuse basement reflections and confirm that the record immediately above basement lacks reflections typical of sedimentary strata. Both observations are consistent with the presence of oceanic crust. Modelling of gravity anomalies suggests the ridge is likely underlain by igneous intrusive rocks displacing mantle rocks, as expected for a volcanic ridge. The seismic data, combined with recently updated multibeam and high-resolution sparker seismic results, further suggest how the evaporite movements have been modulated by basement topography. These results add to our knowledge of the evaporite movements and continent-ocean transition structures in the central Red Sea.

Description: The marine habitat beneath Antarctica’s ice shelves spans ~1.6 million km2, and life in this vast and extreme environment is among Earth’s least accessible, least disturbed and least known, yet likely to be impacted by climate-forced warming and environmental change. Although competition among biota is a fundamental structuring force of ecological communities, hence ecosystem functions and services, nothing was known of competition for resources under ice shelves, until this study. Boreholes drilled through a ~ 200 m thick ice shelf enabled collections of novel sub-ice-shelf seabed sediment which contained fragments of biogenic substrata rich in encrusting (lithophilic) macrobenthos, principally bryozoans – a globally-ubiquitous phylum sensitive to environmental change. Analysis of sub-glacial biogenic substrata, by stereo microscopy, provided first evidence of spatial contest competition, enabling generation of a new range of competition measures for the sub-ice-shelf benthic space. Measures were compared with those of global open-water datasets traversing polar, temperate and tropical latitudes (and encompassing both hemispheres). Spatial competition in sub-ice-shelf samples was found to be higher in intensity and severity than all other global means. The likelihood of sub-ice-shelf competition being intraspecific was three times lower than for open-sea polar continental shelf areas, and competition complexity, in terms of the number of different types of competitor pairings, was two-fold higher. As posited foran enduring disturbance minimum, a specific bryozoan clade was especially competitively dominant in sub-ice shelf samples compared with both contemporary and fossil assemblage records. Overall, spatial competition under an Antarctic ice shelf, as characterised by bryozoan interactions, was strikingly different from that of open- sea polar continental shelf sites, and more closely resembled tropical and temperate latitudes. This study represents the first analysis of sub-ice-shelf macrobenthic spatial competition and provides a new ecological baseline for exploring, monitoring and comparing ecosystem response to environmental change in a warming world.

Description: With changing climate, the boreal forest could potentially migrate north and become threatened by droughts in the south. However, whether larches, the dominant tree species in eastern Siberia, can adapt to novel situations is largely unknown but is crucial for predicting future population dynamics. Exploring variable traits and trait adaptation through inheritance in an individual-based model can improve our understanding and help future projections. We updated the individual-based spatially explicit vegetation model LAVESI (Larix Vegetation Simulator), used for forest predictions in Eastern Siberia, with trait value variation and incorporated inheritance of parental values to their offspring. Forcing the model with both past and future climate projections, we simulated two areas – the expanding northern treeline and a southerly area experiencing drought. While the specific trait of ‘seed weight’ regulates migration, the abstract ‘drought resistance’ protects stands. We show that trait variation with inheritance leads to an increase in migration rate (∼ 3% area increase until 2100). The drought resistance simulations show that, under increasing stress, including adaptive traits leads to larger surviving populations (17% of threatened under RCP 4.5 (Representative Concentration Pathway)). We show that with the increase expected under the RCP 8.5 scenario vast areas (80% of the extrapolated area) of larch forest are threatened and could disappear due to drought as adaptation plays only a minor role under strong warming. We conclude that variable traits facilitate the availability of variants under environmental changes. Inheritance allows populations to adapt to environments and promote successful traits, which leads to populations that can spread faster and be more resilient, provided the changes are not too drastic in both time and magnitude. We show that trait variation and inheritance contribute to more accurate models that can improve our understanding of responses of boreal forests to global change.

Description: While the influence of precession on monsoon at low latitudes through insolation forcing is well-known, the role of obliquity is still debated since its influence on the distribution of incoming solar radiation is small in these regions. In southern Africa, long marine and terrestrial sedimentary records attest of a precessional influence on the South African monsoon at orbital time scale. The obliquity signal is occasionally observed in the geological records although modeling results suggest an influence of precession and obliquity on summer monsoon. Here, we present a record of microscopic charcoal from core MD96-2098 located off Namibia covering the past 184,000 years. Our record of fire activity reveals cyclic changes at frequencies of 23, 58 and 12 kyr−1 and lacks the obliquity signal at 41 kyr−1. Changes in fire over southern Africa are interpreted as shifts in large and intense fires spreading in open-grassland savanna as a result of orbitally-driven changes in rainfall intensity associated with the South African monsoon. We show that, despite the absence of a 41 kyr obliquity imprint, the presence of 23, 58 and 12 kyr−1 frequencies likely stems from a nonlinear response of fire to precipitation controlled by a combination of precession and obliquity frequencies, supporting the influence of obliquity on the South African monsoon.

Description: The cold Last Glacial Maximum, around 20,000 years ago, provides a useful test case for evaluating whether climate models can simulate climate states distinct from the present. However, because of the indirect and uncertain nature of reconstructions of past environmental variables such as sea surface temperature, such evaluation remains ambiguous. Instead, here we evaluate simulations of Last Glacial Maximum climate by relying on the fundamental macroecological principle of decreasing community similarity with increasing thermal distance. Our analysis of planktonic foraminifera species assemblages from 647 sites reveals that the similarity-decay pattern that we obtain when the simulated ice age seawater temperatures are confronted with species assemblages from that time differs from the modern. This inconsistency between the modern temperature dependence of plankton species turnover and the simulations arises because the simulations show globally rather uniform cooling for the Last Glacial Maximum, whereas the species assemblages indicate stronger cooling in the subpolar North Atlantic. The implied steeper thermal gradient in the North Atlantic is more consistent with climate model simulations with a reduced Atlantic meridional overturning circulation. Our approach demonstrates that macroecology can be used to robustly diagnose simulations of past climate and highlights the challenge of correctly resolving the spatial imprint of global change in climate models.

Description: Plant environmental DNA extracted from lacustrine sediments (sedimentary DNA, sedDNA) has been increasingly used to investigate past vegetation changes and human impacts at a high taxonomic resolution. However, the representation of vegetation communities surrounding the lake is still unclear. In this study, we compared plant sedDNA metabarcoding and pollen assemblages from 27 lake surface-sediment samples collected from alpine meadow on the central-eastern Tibetan Plateau to investigate the representation of sedDNA data. In general, the identified components of sedDNA are consistent with the counted pollen taxa and local plant communities. Relative to pollen identification, sedDNA data have higher taxonomic resolution, thus providing a potential approach for reconstructing past plant diversity. The sedDNA signal is strongly influenced by local plants while rarely affected by exogenous plants. Because of the overrepresentation of local plants and PCR bias, the abundance of sedDNA sequence types is very variable among sites, and should be treated with caution when investigating past vegetation cover and climate based on sedDNA data. Our finding suggests that sedDNA analysis can be a complementary approach for investigating the presence/absence of past plants and history of human land-use with higher taxonomic resolution.

Description: Ancient environmental DNA (aeDNA) data are close to enabling insights into past global-scale biodiversity dynamics at unprecedented taxonomic extent and resolution. However, achieving this potential requires solutions that bridge bioinformatics and paleoecoinformatics. Essential needs include support for dynamic taxonomic inferences, dynamic age inferences, and precise stratigraphic depth. Moreover, aeDNA data are complex and heterogeneous, generated by dispersed researcher networks, with methods advancing rapidly. Hence, expert community governance and curation are essential to building high-value data resources. Immediate recommendations include uploading metabarcoding-based taxonomic inventories into paleoecoinformatic resources, building linkages among open bioinformatic and paleoecoinformatic data resources, harmonizing aeDNA processing workflows, and expanding community data governance. These advances will enable transformative insights into global-scale biodiversity dynamics during large environmental and anthropogenic changes.

Description: Relatively little is known about the relationship between the Indian summer monsoon (ISM) and the El Niño-Southern Oscillation (ENSO) on the centennial timescale during the Holocene. We present a well-dated high-resolution X-ray fluorescence (XRF) scanning record from a sediment core from Lake Qionghai on the southeastern Tibetan Plateau, which reveals the impact of ENSO activity on ISM variability. The results indicate a gradual drying of the regional climate on the sub-orbital timescale, which is in broad agreement with ISM changes controlled by Northern Hemisphere summer insolation. Additionally, centennial-scale drought events occurred at around 6230–5740, 4620–4250, 3820–3540, 3210–2440, 2180–1320, and 1000–615 cal yr B.P. and are consistent with enhanced ENSO activity, documenting the occurrence of ENSO-related drought events in the Holocene. Both ISM oscillations and ENSO variability show significant 350-yr, 500-yr, and 800-yr cyclicities, and there is a highly significant negative relationship between the ISM and ENSO at these cyclicities, indicating that a weak ISM was related to increased ENSO intensity, and vice versa. Our findings provide evidence for the modulation of ISM intensity by ENSO variability on the centennial timescale during the Holocene.

Description: Black carbon emitted by incomplete combustion of fossil fuels and biomass has a net warming effect in the atmosphere and reduces the albedo when deposited on ice and snow; accurate knowledge of past emissions is essential to quantify and model associated global climate forcing. Although bottom-up inventories provide historical Black Carbon emission estimates that are widely used in Earth System Models, they are poorly constrained by observations prior to the late 20th century. Here we use an objective inversion technique based on detailed atmospheric transport and deposition modeling to reconstruct 1850 to 2000 emissions from thirteen Northern Hemisphere ice-core records. We find substantial discrepancies between reconstructed Black Carbon emissions and existing bottom-up inventories which do not fully capture the complex spatial-temporal emission patterns. Our findings imply changes to existing historical Black Carbon radiative forcing estimates are necessary, with potential implications for observation-constrained climate sensitivity.