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Multichannel seismic and potential field data shed light on the final rifting stage in the southern South Atlantic. This was associated with major episodes of magmatism during the Early Cretaceous continental break-up. An asymmetrical simple shear-dominated variable strain rifting model is proposed with the margin asymmetry visible in shelf width, amplitude of magnetic anomalies, orientation of break-up-related sedimentary basins and basement slope angle. Along-margin rotation in spreading- and later rifting-direction from north–south to west–east are of great importance for the asymmetries. Such rotational opening may also explain why the southernmost segments of the South Atlantic are magma starved, with a sharp transition to a volcanic-rifted margin type northwards. Interpretation of pre-M5 (c. 130 Ma) magnetic seafloor spreading lineations constrains the timing of excess break-up-related volcanism and transition to ‘normal’ seafloor spreading. Termination of magnetic anomalies within seaward-dipping reflector sequences points towards a deposition of the volcanics from south to north prior to and during the early rift and opening stages. Identification of previously unknown pre-M5 magnetic lineations offshore Argentina completes the lineation pattern in the southern South Atlantic. The oldest magnetic anomaly related to oceanic spreading is M9 (c. 135 Ma). Older anomalies, previously identified as M11 (c. 137 Ma) offshore Cape Town, are related to structural or magnetization variations within seaward-dipping reflector sequences.

The impact of rapid climate change on contemporary human populations is of global concern. To contextualize our understanding of human responses to rapid climate change it is necessary to examine the archeological record during past climate transitions. One episode of abrupt climate change has been correlated with societal collapse at the end of the northwestern European Bronze Age. We apply new methods to interrogate archeological and paleoclimate data for this transition in Ireland at a higher level of precision than has previously been possible. We analyze archeological 14 C dates to demonstrate dramatic population collapse and present high-precision proxy climate data, analyzed through Bayesian methods, to provide evidence for a rapid climatic transition at ca. 750 calibrated years B.C. Our results demonstrate that this climatic downturn did not initiate population collapse and highlight the nondeterministic nature of human responses to past climate change.climate change | demography | prehistory | Bronze Age | radiocarbon dating

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Abstract. High-velocity lower crust (HVLC) and seawarddipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow for a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal cross-sectional areas of HVLC 4 times larger than at the South American margin, a finding that is opposite to the asymmetric distribution of the flood basalts in the Paraná-Etendeka Large Igneous Province. Also, the position of the HVLC with respect to the SDR sequences varies consistently along both margins. Close to the Falkland-Agulhas Fracture Zone in the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below the inner SDR wedges while in the northern area, closer to the Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seaward of the inner SDRs.This challenges the concept of a simple extrusive/intrusive relationship between SDR sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and breakup process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple-shear-dominated extension.