David McGee

Covariant Glacial-Interglacial Dust Fluxes in the Equatorial Pacific and Antarctica

Dust plays a critical role in Earths climate system and serves as a natural source of iron and other micronutrients to remote regions of the ocean. We have generated records of dust deposition over the past 500,000 years at three sites spanning the breadth of the equatorial Pacific Ocean. Equatorial Pacific dust fluxes are highly correlated with global ice volume and with dust fluxes to Antarctica, which suggests that dust generation in interhemispheric source regions exhibited a common response to climate change over late-Pleistocene glacial cycles. Our results provide quantitative constraints on the variability of aeolian iron supply to the equatorial Pacific Ocean and, more generally, on the potential contribution of dust to past climate change and to related changes in biogeochemical cycles.

The Relationship between ITCZ Location and Cross-Equatorial Atmospheric Heat Transport: From the Seasonal Cycle to the Last Glacial Maximum

The authors quantify the relationship between the location of the intertropical convergence zone (ITCZ) and the atmospheric heat transport across the equator (AHTEQ) in climate models and in observations. The observed zonal mean ITCZ location varies from 5.38S in the boreal winter to 7.28N in the boreal summer with an annual mean position of 1.658N while the AHTEQvaries from 2.1 PW northward in the boreal winter to 2.3 PW southward in the boreal summer with an annual mean of 0.1 PW southward. Seasonal variations in the ITCZ location and AHTEQ are highly anticorrelated in the observations and in a suite of state-of-the-art coupledclimatemodelswithregressioncoefficientsof22.78and22.48PW 21 respectively.Itisalsofoundthat seasonal variations in ITCZ location and AHTEQ are well correlated in a suite of slab ocean aquaplanet simulations with varying ocean mixed layer depths. However, the regression coefficient between ITCZ location and AHTEQ decreases with decreasing mixed layer depth as a consequence of the asymmetry that develops between the winter and summer Hadley cells as the ITCZ moves farther off the equator. The authors go on to analyze the annual mean change in ITCZ location and AHTEQ in an ensemble of climate perturbation experiments including the response to CO2 doubling, simulations of the Last Glacial Maximum, and simulations of the mid-Holocene. The shift in the annual average ITCZ location is also strongly anticorrelated with the change in annual mean AHTEQ with a regression coefficient of 23.28 PW 21 , similar to that found over the seasonal cycle.

The Interannual Variability of Tropical Precipitation and Interhemispheric Energy Transport

AbstractThe interannual variability of the location of the intertropical convergence zone (ITCZ) is strongly (R = 0.75) correlated with the atmospheric heat transport across the equator (AHTEQ) over the satellite era (1979–2009). A 1° northward displacement of the ITCZ is associated with 0.34 PW of anomalous AHTEQ from north to south. The AHTEQ and precipitation anomalies are both associated with an intensification of the climatological Hadley cell that is displaced north of the equator. This relationship suggests that the tropical precipitation variability is driven by a hemispheric asymmetry of energy input to the atmosphere at all latitudes by way of the constraint that AHTEQ is balanced by a hemispheric asymmetry in energy input to the atmosphere.A 500-yr coupled model simulation also features strong interannual correlations between the ITCZ location and AHTEQ. The interannual variability of AHTEQ in the model is associated with a hemispheric asymmetry in the top of the atmosphere radiative anomalies ...

Paleodust variability since the Last Glacial Maximum and implications for iron inputs to the ocean

Author(s): Albani, S; Mahowald, NM; Murphy, LN; Raiswell, R; Moore, JK; Anderson, RF; McGee, D; Bradtmiller, LI; Delmonte, B; Hesse, PP; Mayewski, PA | Abstract: ©2016. American Geophysical Union. All Rights Reserved. Changing climate conditions affect dust emissions and the global dust cycle, which in turn affects climate and biogeochemistry. In this study we use observationally constrained model reconstructions of the global dust cycle since the Last Glacial Maximum, combined with different simplified assumptions of atmospheric and sea ice processing of dust-borne iron, to provide estimates of soluble iron deposition to the oceans. For different climate conditions, we discuss uncertainties in model-based estimates of atmospheric processing and dust deposition to key oceanic regions, highlighting the large degree of uncertainty of this important variable for ocean biogeochemistry and the global carbon cycle. We also show the role of sea ice acting as a time buffer and processing agent, which results in a delayed and pulse-like soluble iron release into the ocean during the melting season, with monthly peaks up to ~17 Gg/month released into the Southern Oceans during the Last Glacial Maximum (LGM).

Glacial to Holocene changes in trans-Atlantic Saharan dust transport and dust-climate feedbacks

Variations in long-range Saharan dust transport may have amplified past Atlantic ITCZ and West African monsoon changes. Saharan mineral dust exported over the tropical North Atlantic is thought to have significant impacts on regional climate and ecosystems, but limited data exist documenting past changes in long-range dust transport. This data gap limits investigations of the role of Saharan dust in past climate change, in particular during the mid-Holocene, when climate models consistently underestimate the intensification of the West African monsoon documented by paleorecords. We present reconstructions of African dust deposition in sediments from the Bahamas and the tropical North Atlantic spanning the last 23,000 years. Both sites show early and mid-Holocene dust fluxes 40 to 50% lower than recent values and maximum dust fluxes during the deglaciation, demonstrating agreement with records from the northwest African margin. These quantitative estimates of trans-Atlantic dust transport offer important constraints on past changes in dust-related radiative and biogeochemical impacts. Using idealized climate model experiments to investigate the response to reductions in Saharan dust’s radiative forcing over the tropical North Atlantic, we find that small (0.15°C) dust-related increases in regional sea surface temperatures are sufficient to cause significant northward shifts in the Atlantic Intertropical Convergence Zone, increased precipitation in the western Sahel and Sahara, and reductions in easterly and northeasterly winds over dust source regions. Our results suggest that the amplifying feedback of dust on sea surface temperatures and regional climate may be significant and that accurate simulation of dust’s radiative effects is likely essential to improving model representations of past and future precipitation variations in North Africa.

Extraterrestrial He in Sediments: From Recorder of Asteroid Collisions to Timekeeper of Global Environmental Changes

Most 3He in deep-sea sediments is derived from fine-grained extraterrestrial matter known as interplanetary dust particles (IDPs). These particles, typically <50 μm in diameter, are sufficiently small to retain solar wind-implanted He with high 3He/4He ratios during atmospheric entry heating. This extraterrestrial 3He (3HeET) is retained in sediments for geologically long durations, having been detected in sedimentary rocks as old as 480 Ma. As a tracer of fine-grained extraterrestrial material, 3HeET offers unique insights into solar system events associated with increased IDP fluxes, including asteroid break-up events and comet showers. Studies have used 3HeET to identify IDP flux changes associated with a Miocene asteroid break-up event and a likely comet shower in the Eocene. During much of the Cenozoic, 3HeET fluxes have remained relatively constant over million-year timescales, enabling 3HeET to be used as a constant flux proxy for calculating sedimentary mass accumulation rates and constraining sedimentary age models. We review studies employing 3HeET-based accumulation rates to estimate the duration of carbonate dissolution events associated with the K/Pg boundary and Paleocene-Eocene Thermal Maximum. Additionally, 3HeET has been used to quantify sub-orbital variability in fluxes of paleoproductivity proxies and windblown dust. In order to better interpret existing records and guide the application of 3HeET in novel settings, future work requires constraining the carrier phase(s) of 3HeET responsible for long-term retention in sediments, better characterizing the He isotopic composition of the terrigenous end-member, and understanding why observed extraterrestrial 3He fluxes do not match the predicted variability of IDP accretion rate over orbital timescales.

Changes in biological productivity along the northwest African margin over the past 20,000 years

The intertropical convergence zone and the African monsoon system are highly sensitive to climate forcing at orbital and millennial timescales. Both systems influence the strength and direction of the trade winds along northwest Africa and thus directly impact coastal upwelling. Sediment cores from the northwest African margin record upwelling-related changes in biological productivity connected to changes in regional and hemispheric climate. We present records of 230Th-normalized biogenic opal and Corg fluxes using a meridional transect of four cores from 19°N–31°N along the northwest African margin to examine changes in paleoproductivity since the last glacial maximum. We find large changes in biogenic fluxes synchronous with changes in eolian fluxes calculated using end-member modeling, suggesting that paleoproductivity and dust fluxes were strongly coupled, likely linked by changes in wind strength. Opal and Corg fluxes increase at all sites during Heinrich Stadial 1 and the Younger Dryas, consistent with an overall intensification of the trade winds, and changes in the meridional flux gradient indicate a southward wind shift at these times. Biogenic fluxes were lowest, and the meridional flux gradients were weakest during the African Humid Period when the monsoon was invigorated due to precessional changes, with greater rainfall and weaker trade winds over northwest Africa. These results expand the spatial coverage of previous paleoproxy studies showing similar changes, and they provide support for modeling studies showing changes in wind strength and direction consistent with increased upwelling during abrupt coolings and decreased upwelling during the African Humid Period.

Thorium distributions in high‐ and low‐dust regions and the significance for iron supply

Thorium and uranium isotopes (232Th, 230Th, 238U, and 234U) were investigated to refine their use for estimating mineral dust deposition and Fe delivery to the ocean. U concentrations and isotope ratios were consistent with conservative behavior and can safely be described using published U-salinity relationships and global average seawater isotopic composition. Near Barbados, waters affected by the Amazon outflow contained elevated 232Th. This signals one region where the thorium-dust method is inaccurate because of a confounding continental input. Dissolved 232Th fluxes in this region suggest that Amazonian Fe supply to the adjacent open ocean is much larger than local atmospheric deposition. The colloidal content of dissolved Th south of Bermuda was found to be quite small (2–6%), similar to that found north of Hawaii, despite the order of magnitude higher dust deposition in the Atlantic. This finding supports the assumption that dissolved 232Th and 230Th are scavenged at the same rate despite their different sources and also sheds light on the increase of dissolved 232Th fluxes with integrated depth. Outside the region influenced by Amazon River waters, dissolved 232Th fluxes are compared with Bermudan aerosol Fe deposition to estimate that fractional Th solubility is around 20% in this region. Finally, new dissolved and soluble Fe, Mn, and Cr data from the subtropical North Pacific support the idea that Fe concentrations in the remote ocean are highly buffered, whereas 232Th has a larger dynamic range between high- and low-dust regions.

Archeometria e studio ambientale di un probabile sito di culto del Paleolitico superiore nella Grotta del Fiume a Frasassi (Marche, Italia)

Archeometry and environmental study of a probable late Paleolithic cult site in the Grotta del Fiume cave at Frasassi (Marche region of Italy).

Linking Glacial‐Interglacial States to Multiple Equilibria of Climate

Glacial-Interglacial cycles are often described as an amplified global response of the climate to perturbations in solar radiation caused by oscillations of Earths orbit. However, it remains unclear whether internal feedbacks are large enough to account for the radically different Glacial and Interglacial states. Here we provide support for an alternative view: Glacial-Interglacial states are multiple equilibria of the climate system which exist for the same external forcing. We show that such multiple equilibria resembling Glacial and Interglacial states can be found in a complex coupled General Circulation Model of the ocean-atmosphere-sea ice system. The multiple states are sustained by ice-albedo feedback modified by ocean heat transport and are not caused by the bi-stability of the oceans overturning circulation. In addition, expansion/contraction of the Southern Hemisphere ice pack over regions of upwelling, regulating outgassing of CO