We investigate the incidence of major mergers creating massive (Mstar > 1011 M⊙) galaxies in present‐day (z≤ 0.12) groups and clusters. Using a volume‐limited sample of 845 groups with dark matter halo masses above 2.5 × 1013 M⊙, we isolate 221 galaxy pairs with ≤1.5 r‐band magnitude differences, ≤30 kpc projected separations and combined masses above 1011 M⊙. We fit the r‐band images of each pair as the line‐of‐sight projection of symmetric models and identify 38 mergers by the presence of residual asymmetric structure associated with both progenitors, such as non‐concentric isophotes, broad and diffuse tidal tails and dynamical friction wakes. In other words, at the resolution and sensitivity of the Sloan Digital Sky Survey (SDSS), 16 per cent of massive major pairs in dense environments have mutual tidal interaction signatures; relying on automated searches of major pairs from the SDSS spectroscopic galaxy sample will result in missing 70 per cent of these mergers owing to spectroscopic incompleteness in high‐density regions. We find that 90 per cent of these mergers are between two nearly equal‐mass progenitors with red‐sequence colours and centrally concentrated morphologies, in agreement with numerical simulations that predict that an important mechanism for the formation of massive elliptical galaxies is the dissipationless (gas‐poor or so‐called dry) major merging of spheroid‐dominated galaxies. We identify seven additional massive mergers with disturbed morphologies and semiresolved double nuclei; thus, 1.5 ± 0.2 per cent of Mstar≥ 5 × 1010 M⊙ galaxies in large groups are involved in the major merger assembly of massive galaxies. Mergers at the centres of these groups are more common than between two satellites, but both types are morphologically indistinguishable and we tentatively conclude that the latter are likely located at the dynamical centres of large subhaloes that have recently been accreted by their host halo. Based on reasonable assumptions, the centres of group and cluster‐sized haloes are gaining stellar mass at a rate of 2–9 per cent per Gyr on average. Our results indicate that the massive end of the galaxy population continues to evolve hierarchically at a measurable level, and that massive mergers are more likely to occur in large galaxy groups than in massive clusters.

Both instrumental data analyses and coupled ocean-atmosphere models indicate that Atlantic meridional overturning circulation (AMOC) variability is tightly linked to abrupt tropical North Atlantic (TNA) climate change through both atmospheric and oceanic processes. Although a slowdown of AMOC results in an atmospheric-induced surface cooling in the entire TNA, the subsurface experiences an even larger warming because of rapid reorganizations of ocean circulation patterns at intermediate water depths. Here, we reconstruct high-resolution temperature records using oxygen isotope values and Mg/Ca ratios in both surface- and subthermocline-dwelling planktonic foraminifera from a sediment core located in the TNA over the last 22 ky. Our results show significant changes in the vertical thermal gradient of the upper water column, with the warmest subsurface temperatures of the last deglacial transition corresponding to the onset of the Younger Dryas. Furthermore, we present new analyses of a climate model simulation forced with freshwater discharge into the North Atlantic under Last Glacial Maximum forcings and boundary conditions that reveal a maximum subsurface warming in the vicinity of the core site and a vertical thermal gradient change at the onset of AMOC weakening, consistent with the reconstructed record. Together, our proxy reconstructions and modeling results provide convincing evidence for a subsurface oceanic teleconnection linking high-latitude North Atlantic climate to the tropical Atlantic during periods of reduced AMOC across the last deglacial transition.

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Much uncertainty exists about the state of the oceanic and atmospheric circulation in the tropical Pacific over the last glacial cycle. Studies have been hampered by the fact that sediment cores suitable for study were concentrated in the western and eastern parts of the tropical Pacific, with little information from the central tropical Pacific. Here we present information from a suite of sediment cores collected from the Line Islands Ridge in the central tropical Pacific, which show sedimentation rates and stratigraphies suitable for paleoceanographic investigations. Based on the radiocarbon and oxygen isotope measurements on the planktonic foraminifera Globigerinoides ruber, we construct preliminary age models for selected cores and show that the gradient in the oxygen isotope ratio of G. ruber between the equator and 8°N is enhanced during glacial stages relative to interglacial stages. This stronger gradient could reflect enhanced equatorial cooling (perhaps reflecting a stronger Walker circulation) or an enhanced salinity gradient (perhaps reflecting increased rainfall in the central tropical Pacific).