Roberts et al. (1) state that we found no evidence of climate change at multidecadal to millennial timescales following deposition of the Youngest Toba Tuff (YTT) in Lake Malawi (2). However, we examined smear slides at a 2-mm interval, corresponding to subdecadal resolution, and X-ray fluorescence scans run at 200-µm intervals correspond to subannual resolution. We observed no obvious change in sediment composition or Fe/Ti ratio, suggesting that no thermally driven overturn of the water column occurred following the Toba supereruption.
Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth's climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO(2) and CH(4) to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the Atlantic meridional overturning circulation.
We analyzed a varved sequence of sediment from a 350‐m depth in the north basin of Lake Malawi, East Africa, for the size distribution of the sortable silt fraction (10‐64 µm). Mean size of the sortable silt (mean SS) varies measurably in sediments spanning the past 650 yr and covaries with the mass accumulation rate of terrigenous silt and clay (TMAR) over much of the interval. Most of the silt and clay is delivered to the offshore basin in benthic nepheloid plumes of unknown duration and frequency. TMAR appears to be related to annual rainfall (which is related to the North Atlantic Oscillation) because it roughly tracks the historical record of lake level that extends back to 1860. Mean SS should be related to density or thickness of underflow, thus related to resuspension intensity or river flood loading. It also tracks lake level and regional wind strength as determined by National Center for Environmental Prediction (NCEP) Reanalysis.
We present a synthesis of available estimates of primary production, organic carbon burial, and lake‐atmosphere carbon dioxide exchange data for large lakes of the world. All three fluxes showed significant relationships with latitude and related climate variables, with lower production, higher evasion of carbon dioxide, and higher burial efficiency at higher latitudes. There was no relationship between raw organic carbon mass accumulation rates and latitude. Our estimates suggest that an order of magnitude more carbon is lost to the atmosphere by evasion than is buried in sediments at a global scale, with total global production, evasion, and burial fluxes of approximately 250, 90, and 7 Tg C yr −1 . Finally, the data suggest a trend from autotrophy in low‐latitude large lakes to heterotrophy and increasing reliance on allochthonous carbon sources in lakes at higher latitudes.
Research Article| December 01, 1984 Effects of bottom currents and fish on sedimentation in a deep-water, lacustrine environment THOMAS C. JOHNSON; THOMAS C. JOHNSON 1Duke University Marine Laboratory, Beaufort, North Carolina 28516 Search for other works by this author on: GSW Google Scholar JOHN D. HALFMAN; JOHN D. HALFMAN 1Duke University Marine Laboratory, Beaufort, North Carolina 28516 Search for other works by this author on: GSW Google Scholar WILLIAM H. BUSCH; WILLIAM H. BUSCH 2Department of Earth Sciences, University of New Orleans, New Orleans, Louisiana 70148 Search for other works by this author on: GSW Google Scholar ROGER D. FLOOD ROGER D. FLOOD 3Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1984) 95 (12): 1425–1436. https://doi.org/10.1130/0016-7606(1984)95<1425:EOBCAF>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation THOMAS C. JOHNSON, JOHN D. HALFMAN, WILLIAM H. BUSCH, ROGER D. FLOOD; Effects of bottom currents and fish on sedimentation in a deep-water, lacustrine environment. GSA Bulletin 1984;; 95 (12): 1425–1436. doi: https://doi.org/10.1130/0016-7606(1984)95<1425:EOBCAF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Sands and muds accumulating under the influence of apparently strong bottom currents in a deep-water environment of Lake Superior were studied in detail using 3.5-kHz echo sounding; side-scan SONAR; piston, gravity, and box coring; and lake-floor photography. The study area includes a well-defined slope and basin environment. Bottom currents maintain a scoured trough, 20 m deep and 2 km wide, at the base of the slope and modify the shapes of large ringlike depressions that are common in many regions of Lake Superior. Side-scan SONAR records reveal lineations subparallel to the direction of bottom-current flow that are interpreted in different areas to be sand ribbons, slight depressions, or depositional furrows.The sediments range from silty sand in the scoured trough to sandy clays farther offshore. Radiocarbon dating and 210Pb dating show the sediments to be postglacial rather than relic glacial-lacustrine sediments. Grain size distributions are affected by bottom currents and a nearby source of sand. Some sedimentary structures, including plane laminations and interbedded sands and muds, are due to bottom currents. Fish create dish laminations, however, that often obscure the effects of the bottom currents. Fish and fish-scour depressions are common in lake-floor photographs, whereas evidence for bottom currents is not. Biological activity therefore appears to erase traces of intermittent currents. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Research Article| September 01, 2007 Wet and arid phases in the southeast African tropics since the Last Glacial Maximum Isla S. Castañeda; Isla S. Castañeda 1Large Lakes Observatory, University of Minnesota Duluth, Duluth, Minnesota 55812, USA Search for other works by this author on: GSW Google Scholar Josef P. Werne; Josef P. Werne 2Large Lakes Observatory and Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, USA Search for other works by this author on: GSW Google Scholar Thomas C. Johnson Thomas C. Johnson 3Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, Duluth, Minnesota 55812, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Isla S. Castañeda 1Large Lakes Observatory, University of Minnesota Duluth, Duluth, Minnesota 55812, USA Josef P. Werne 2Large Lakes Observatory and Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, USA Thomas C. Johnson 3Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, Duluth, Minnesota 55812, USA Publisher: Geological Society of America Received: 22 Mar 2007 Revision Received: 26 Apr 2007 Accepted: 01 May 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2007) 35 (9): 823–826. https://doi.org/10.1130/G23916A.1 Article history Received: 22 Mar 2007 Revision Received: 26 Apr 2007 Accepted: 01 May 2007 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Isla S. Castañeda, Josef P. Werne, Thomas C. Johnson; Wet and arid phases in the southeast African tropics since the Last Glacial Maximum. Geology 2007;; 35 (9): 823–826. doi: https://doi.org/10.1130/G23916A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Plant leaf wax carbon isotopes provide a record of C3 versus C4 vegetation, a sensitive indicator of aridity, from the southeast African tropics since the Last Glacial Maximum. Wet and arid phases in southeast Africa were in phase with conditions in the global tropics from 23 to 11 ka, but at the start of the Holocene these relationships ended and an antiphase relationship prevailed. The abrupt switch from in phase to out of phase conditions may partially be attributed to a southward displacement of the Intertropical Convergence Zone (ITCZ) during the last glacial. Southward displacements of the ITCZ are also linked to arid conditions in southeast Africa during the Younger Dryas and the Little Ice Age. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Seventeen box cores from Lake Superior were analyzed for total organic carbon (TOC), porosity, and 210 Pb activity. The data were used to determine relationships among bulk sedimentation rates, TOC sedimentation rates, and TOC degradation rates with depth in the core. Results are compared with predictive equations for TOC sedimentation rates and degradation rates in the oceans and show some striking similarities between the behavior of TOC in Lake Superior and in hemipelagic and pelagic marine environments.
The role of the tropics in global climate change is actively debated, particularly in regard to the timing and magnitude of thermal and hydrological response. Continuous, high‐resolution temperature records through the Last Glacial Maximum (LGM) from tropical oceans have provided much insight but surface temperature reconstructions do not exist from tropical continental environments. Here we used the TEX 86 paleotemperature proxy to reconstruct mean annual lake surface temperatures through the Last Glacial Maximum (LGM) in Lake Malawi, East Africa (9°–14°S). We find a ∼3.5°C overall warming since the LGM, with temperature reversals of ∼2°C during the Younger Dryas (12.5 ka BP) and at 8.2 ka BP. Maximum Holocene temperatures of ∼29°C were found at 5 ka BP, a period preceding severe drought in Africa. These results suggest a substantial thermal response of southeastern tropical Africa to deglaciation and to varying conditions during the Holocene.
