[1] In order to test the sensitivity of marine primary productivity in the midlatitude open ocean North Atlantic to changes in the Atlantic Meridional Overturning Circulation (AMOC), we investigated two spliced sediment cores from a site south of the Azores Islands at the northern rim of the North Atlantic subtropical gyre. For this purpose we analyzed coccolithophore assemblages, diatom abundances, alkenones and conducted X-ray fluorescence (XRF) core scanning. During times of reduced AMOC, especially during Heinrich event 1 (H1) and the Younger Dryas, we observe a strong increase in productivity as evidenced by high coccolith accumulation rates, high alkenone concentrations/accumulation rates, high Ba/Ti-ratios, high abundances of diatoms and low abundances of F. profunda. The increased productivity is partly caused by a more southern position of the Azores Front (AzF), and hence by a less northward extension of the subtropical gyre, as deduced from high abundances of the temperate coccolithophore species G. muellerae and low abundances of subtropical species (Oolithotus spp., Umbellosphaera spp., Umbilicosphaera spp.). However, to explain the full range of the observed productivity increase, other factors like increased westerly winds and advection of nutrient-rich surface waters have also to be considered. Because this pattern can also be observed in other sediment cores from the midlatitude North Atlantic, we propose that during times of reduced AMOC there has been a band of strongly increased productivity across the North Atlantic at the northern rim of the contracted subtropical gyre, which partly counteracts the decreased organic carbon pump in the high northern latitudes.Citation: Schwab, C., H. Kinkel, M. Weinelt, and J. Repschläger (2012), Coccolithophore paleoproductivity and ecology response to deglacial and Holocene changes in the Azores Current System, Paleoceanography, 27, PA3210,
The North Atlantic subtropical gyre (STG) circulates warm waters between 10 and 40°N and is a potential area of heat storage during periods of reduced North Atlantic Meridional Overturning Circulation (AMOC), when warm salt-rich waters are retained in the subtropics. In this study, we investigated multicentennial to millennial scale changes in subtropical North Atlantic hydrography in response to AMOC changes during the last deglaciation and early Holocene, using sediment cores MD08-3180 and GEOFAR KF16. The coring site (38°N) is situated near the boundary between transitional eastern North Atlantic waters and STG waters that is formed by the Azores Front. Hydrographic changes are reconstructed using new stable isotope data of benthic and subsurface dwelling planktonic foraminifera, Mg/Ca measurements on planktonic foraminifera, and planktonic foraminifera abundances that are supplemented with published sea surface temperature and stable isotope data. These multiproxy data indicate a close coupling between the latitudinal position of the northern STG boundary and deglacial AMOC modes. During weak AMOC phases (Heinrich event 1, Younger Dryas (YD), 8.2 ka event), Northern Hemisphere subpolar water reached down to the northern STG boundary, displacing the boundary southward. During the Bølling-Allerød warm period, a strong warming trend of the subtropical region to 19°C is observed. A cooling of the sea surface temperature by 6°C during the YD is accompanied by ongoing northward transport of warm subsurface water that might have contributed to the restart of AMOC.
Reconstruction of Skagerrak deep-water renewal is used to assess regional changes in winter thermal conditions over the past 6800 years. Changes in winter climate conditions from the Skagerrak region are in turn linked to shifts in Holocene large-scale atmospheric circulation patterns prevailing over northern Europe. We use Melonis barleeanus Mg/Ca from two sediment cores in the central Skagerrak to reconstruct temperature of Skagerrak intermediate water, representing the warm season temperature variability, and deep water, for monitoring Skagerrak deep-water renewal, reflecting the winter temperature variability. In addition, M. barleeanus δ18O is used from the deeper core to reconstruct salinity, also monitoring the deep-water renewal. Our results show that the Skagerrak deep-water experienced phases of particularly enhanced renewal during the mid-Holocene reflecting severe winter conditions, followed by a general shift to reduced renewal as a consequence of milder winter conditions over the North Sea around 3500 cal. yr BP. The late-Holocene shift was most likely related to the onset of a regime with intensified winter westerly winds directed toward northern Europe and an increased inflow of North Atlantic water into the Skagerrak–North Sea reflecting more maritime climate conditions. On millennial scale, cold phases in our deep-water records match with low winter precipitation phases in western Norway. They are associated with distinct increases in ice rafted debris (IRD) in North Atlantic sediments, suggesting that phases of iceberg discharge in the Atlantic were associated with cold and dry winter conditions over northern Europe. Interestingly, the cold event centered around 5900 cal. yr BP appears to be only associated with winter variability, while the following one at 4200 cal. yr BP is documented in our winter record, as well as in records related to warmer seasons.
We present the discovery of TOI-5205b, a transiting Jovian planet orbiting a solar metallicity M4V star, which was discovered using Transiting Exoplanet Survey Satellite photometry and then confirmed using a combination of precise radial velocities, ground-based photometry, spectra, and speckle imaging. TOI-5205b has one of the highest mass ratios for M-dwarf planets, with a mass ratio of almost 0.3%, as it orbits a host star that is just 0.392 ± 0.015 M ⊙. Its planetary radius is 1.03 ± 0.03 R J, while the mass is 1.08 ± 0.06 M J. Additionally, the large size of the planet orbiting a small star results in a transit depth of ∼7%, making it one of the deepest transits of a confirmed exoplanet orbiting a main-sequence star. The large transit depth makes TOI-5205b a compelling target to probe its atmospheric properties, as a means of tracing the potential formation pathways. While there have been radial-velocity-only discoveries of giant planets around mid-M dwarfs, this is the first transiting Jupiter with a mass measurement discovered around such a low-mass host star. The high mass of TOI-5205b stretches conventional theories of planet formation and disk scaling relations that cannot easily recreate the conditions required to form such planets.
We present the discovery of a new Jovian-sized planet, TOI-3757 b, the lowest-density transiting planet known to orbit an M dwarf (M0V). This planet was discovered around a solar-metallicity M dwarf, using Transiting Exoplanet Survey Satellite photometry and confirmed with precise radial velocities from the Habitable-zone Planet Finder (HPF) and NEID. With a planetary radius of 12.0 − 0.5 + 0.4 R ⊕ and mass of 85.3 − 8.7 + 8.8 M ⊕, not only does this object add to the small sample of gas giants (∼10) around M dwarfs, but also its low density ( ρ = 0.27 − 0.04 + 0.05 g cm−3) provides an opportunity to test theories of planet formation. We present two hypotheses to explain its low density; first, we posit that the low metallicity of its stellar host (∼0.3 dex lower than the median metallicity of M dwarfs hosting gas giants) could have played a role in the delayed formation of a solid core massive enough to initiate runaway accretion. Second, using the eccentricity estimate of 0.14 ± 0.06, we determine it is also plausible for tidal heating to at least partially be responsible for inflating the radius of TOI-3757b b. The low density and large scale height of TOI-3757 b makes it an excellent target for transmission spectroscopy studies of atmospheric escape and composition (transmission spectroscopy measurement of ∼ 190). We use HPF to perform transmission spectroscopy of TOI-3757 b using the helium 10830 Å line. Doing this, we place an upper limit of 6.9% (with 90% confidence) on the maximum depth of the absorption from the metastable transition of He at ∼10830 Å, which can help constraint the atmospheric mass-loss rate in this energy-limited regime.
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