scholarly journals The Mysterious 14C Decline

Radiocarbon ◽  
2009 ◽  
Vol 51 (1) ◽  
pp. 109-119 ◽  
Author(s):  
Wallace Broecker
Keyword(s):  
Age Differences ◽  
Radiocarbon Dating ◽  
Progressive Increase ◽  
Time Interval ◽  
Last Deglaciation ◽  
Polar Ice ◽  
Radiocarbon Age ◽  
Surface Ocean ◽  
The Past ◽  
The Last Deglaciation

Fundamental to the field of radiocarbon dating is not only the establishment of the temporal record of the calendar age-radiocarbon age offsets but also the development of an understanding of their cause. Although part of the decline in the magnitude of this offset over the past 40,000 can be explained by a drop in 14C production rate associated with a progressive increase in the strength of the Earth's magnetic shielding, it is clear that changes in the distribution of 14C among the Earth's active carbon reservoirs are also required. In particular, the steep 15% decline in the 14C to C ratio in atmospheric CO2 and surface ocean ΣCO2, which occurred in a 3 kyr-duration interval marking the onset of the last deglaciation, appears to require that a very large amount (at least 5000 gigatons) of 14C-deficient carbon was transferred to or within the ocean during this time interval. As no chemical or stable isotope anomaly associated with this injection appears in either the marine sediment or polar ice records, this injection must involve a transfer within the ocean (i.e. a mixing of 2 ocean reservoirs, one depleted in 14C and the other enriched in 14C). Although evidence for the existence of a salt-stabilized glacial-age abyssal ocean reservoir exists, a search based on benthic-planktic age differences and 13C measurements appears to place a limit on its size well below that required to account for the steep 14C decline.

scholarly journals The Meridional Shift of the Midlatitude Westerlies over Arid Central Asia during the Past 21 000 Years Based on the TraCE-21ka Simulations

2020 ◽  
Vol 33 (17) ◽  
pp. 7455-7478
Author(s):  
Nanxuan Jiang ◽  
Qing Yan ◽  
Zhiqing Xu ◽  
Jian Shi ◽  
Ran Zhang
Keyword(s):  
Indian Ocean ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Last Deglaciation ◽  
The Past ◽  
The North ◽  
External Forcings ◽  
The Indian Ocean ◽  
The Last Deglaciation ◽  
The North Atlantic

AbstractTo advance our knowledge of the response of midlatitude westerlies to various external forcings, we investigate the meridional shift of midlatitude westerlies over arid central Asia (ACA) during the past 21 000 years, which experienced more varied forcings than the present day based on a set of transient simulations. Our results suggest that the evolution of midlatitude westerlies over ACA and driving factors vary with time and across seasons. In spring, the location of midlatitude westerlies over ACA oscillates largely during the last deglaciation, driven by meltwater fluxes and continental ice sheets, and then shows a long-term equatorward shift during the Holocene controlled by orbital insolation. In summer, orbital insolation dominates the meridional shift of midlatitude westerlies, with poleward and equatorward migration during the last deglaciation and the Holocene, respectively. From a thermodynamic perspective, variations in zonal winds are linked with the meridional temperature gradient based on the thermal wind relationship. From a dynamic perspective, variations in midlatitude westerlies are mainly induced by anomalous sea surface temperatures over the Indian Ocean through the Matsuno–Gill response and over the North Atlantic Ocean by the propagation of Rossby waves, or both, but their relative importance varies across forcings. Additionally, the modeled meridional shift of midlatitude westerlies is broadly consistent with geological evidence, although model–data discrepancies still exist. Overall, our study provides a possible scenario for a meridional shift of midlatitude westerlies over ACA in response to various external forcings during the past 21 000 years and highlights important roles of both the Indian Ocean and the North Atlantic Ocean in regulating Asian westerlies, which may shed light on the behavior of westerlies in the future.

scholarly journals Global acceleration in rates of vegetation change over the past 18,000 years

Science ◽  
2021 ◽  
Vol 372 (6544) ◽  
pp. 860-864
Author(s):  
Ondřej Mottl ◽  
Suzette G. A. Flantua ◽  
Kuber P. Bhatta ◽  
Vivian A. Felde ◽  
Thomas Giesecke ◽  
...  
Keyword(s):  
Vegetation Change ◽  
Terrestrial Ecosystems ◽  
Past Century ◽  
Last Deglaciation ◽  
The Past ◽  
Rates Of Change ◽  
Biodiversity Change ◽  
Human Effects ◽  
The Last Deglaciation ◽  
Global Vegetation

Global vegetation over the past 18,000 years has been transformed first by the climate changes that accompanied the last deglaciation and again by increasing human pressures; however, the magnitude and patterns of rates of vegetation change are poorly understood globally. Using a compilation of 1181 fossil pollen sequences and newly developed statistical methods, we detect a worldwide acceleration in the rates of vegetation compositional change beginning between 4.6 and 2.9 thousand years ago that is globally unprecedented over the past 18,000 years in both magnitude and extent. Late Holocene rates of change equal or exceed the deglacial rates for all continents, which suggests that the scale of human effects on terrestrial ecosystems exceeds even the climate-driven transformations of the last deglaciation. The acceleration of biodiversity change demonstrated in ecological datasets from the past century began millennia ago.

scholarly journals Causes and timing of recurring subarctic Pacific hypoxia

2021 ◽  
Vol 7 (23) ◽  
pp. eabg2906
Author(s):  
Karla P. Knudson ◽  
Ana Christina Ravelo ◽  
Ivano W. Aiello ◽  
Christina P. Knudson ◽  
Michelle K. Drake ◽  
...  
Keyword(s):  
Laminated Sediments ◽  
Last Deglaciation ◽  
Glacial Cycles ◽  
Sea Levels ◽  
Subarctic Pacific ◽  
High Productivity ◽  
The Past ◽  
Continental Shelves ◽  
Pacific Studies ◽  
The Last Deglaciation

Several North Pacific studies of the last deglaciation show hypoxia throughout the ocean margins and attribute this phenomenon to the effects of abrupt warming and meltwater inputs. Yet, because of the lack of long records spanning multiple glacial cycles and deglaciation events, it is unclear whether deoxygenation was a regular occurrence of warming events and whether deglaciation and/or other conditions promoted hypoxia throughout time. Here, subarctic Pacific laminated sediments from the past 1.2 million years demonstrate that hypoxic events recurred throughout the Pleistocene as episodes of highly productive phytoplankton growth and were generally associated with interglacial climates, high sea levels, and enhanced nitrate utilization—but not with deglaciations. We suggest that hypoxia was typically stimulated by high productivity from iron fertilization facilitated by redox-remobilized iron from flooded continental shelves.

scholarly journals CH<sub>4</sub> and N<sub>2</sub>O fluctuations during the penultimate deglaciation

2021 ◽  
Vol 17 (4) ◽  
pp. 1627-1643
Author(s):  
Loïc Schmidely ◽  
Christoph Nehrbass-Ahles ◽  
Jochen Schmitt ◽  
Juhyeong Han ◽  
Lucas Silva ◽  
...  
Keyword(s):  
Ice Core ◽  
Last Deglaciation ◽  
Glacial Cycle ◽  
Last Glacial Period ◽  
Modes Of Variability ◽  
The Past ◽  
Present Composite ◽  
The Last Deglaciation ◽  
Ch4 And N2o ◽  
The Last Glacial

Abstract. Deglaciations are characterized by the largest natural changes in methane (CH4) and nitrous oxide (N2O) concentrations of the past 800 000 years. Reconstructions of millennial- to centennial-scale variability within these periods are mostly restricted to the last deglaciation. In this study, we present composite records of CH4 and N2O concentrations from the EPICA Dome C ice core covering the penultimate deglaciation at temporal resolutions of ∼100 years. Our data permit the identification of centennial-scale fluctuations during the transition from glacial to interglacial levels. At ∼134 000 and ∼129 000 years before present (hereafter ka), both CH4 and N2O increased on centennial timescales. These abrupt rises are similar to the fluctuations associated with the Dansgaard–Oeschger events identified in the last glacial period. In addition, gradually rising N2O levels at ∼130 ka resemble a pattern of increasing N2O concentrations on millennial timescales characterizing the later part of Heinrich stadials. Overall, the events in CH4 and N2O during the penultimate deglaciation exhibit modes of variability that are also found during the last deglaciation and glacial cycle, suggesting that the processes leading to changes in emission during the transitions were similar but their timing differed.

scholarly journals The penultimate deglaciation: protocol for PMIP4 transient numerical simulations between 140 and 127 ka

2018 ◽  
Author(s):  
Laurie Menviel ◽  
Emilie Capron ◽  
Aline Govin ◽  
Andrea Dutton ◽  
Lev Tarasov ◽  
...  
Keyword(s):  
Greenhouse Gas ◽  
Working Group ◽  
Ice Sheets ◽  
Climate System ◽  
Time Interval ◽  
Last Deglaciation ◽  
Last Interglacial ◽  
Transient Simulations ◽  
The Last Deglaciation ◽  
Continental Ice Sheets

Abstract. The penultimate deglaciation (~ 138–128 thousand years before present, hereafter ka) is the transition from the penultimate glacial maximum to the Last Interglacial (LIG, ~ 129–116 ka). The LIG stands out as one of the warmest interglacials of the last 800 ka, with high-latitude temperature warmer than today and global sea level likely higher by at least 6 meters. The LIG therefore receives ever-growing attention, in particular to identify mechanisms and feedbacks responsible for such regional warmth that is comparable to that expected before 2100. Considering the transient nature of the Earth system, the LIG climate and ice-sheets evolution were certainly influenced by the changes occurring during the penultimate deglaciation. It is thus important to investigate the climate and environmental response to the large changes in boundary conditions (i.e. orbital configuration, atmospheric greenhouse gas concentrations, ice sheet geometry) occurring during this time interval. A deglaciation working group has recently been set up as part of the Paleoclimate Modelling Intercomparison Project (PMIP) phase 4, with a protocol to perform transient simulations of the last deglaciation (19–11 ka). Similar to the last deglaciation, the disintegration of continental ice-sheets during the penultimate deglaciation led to significant changes in the oceanic circulation during Heinrich Stadial 11 (~ 136–129 ka). However, the two deglaciations bear significant differences in magnitude and temporal evolution of climate and environmental changes. Here, as part of the PAGES-PMIP working group on Quaternary Interglacials, we propose a protocol to perform transient simulations of the penultimate deglaciation to complement the PMIP4 effort. This design includes time-varying changes in orbital forcing, greenhouse gas concentrations, continental ice-sheets as well as freshwater input from the disintegration of continental ice-sheets. This experiment is designed to assess the coupled response of the climate system to all forcings. Additional sensitivity experiments are proposed to evaluate the response to each forcing. Finally, a selection of paleo records representing different parts of the climate system is presented, providing an appropriate benchmark for upcoming model-data comparisons across the penultimate deglaciation.

History of the endangered thread-leaved sundew (Drosera filiformis) in southern Nova Scotia

2005 ◽  
Vol 83 (1) ◽  
pp. 14-21 ◽  
Author(s):  
Mark Landry ◽  
Les C Cwynar
Keyword(s):  
Nova Scotia ◽  
Residence Time ◽  
Radiocarbon Dating ◽  
Plant Macrofossil ◽  
Radiocarbon Age ◽  
Macrofossil Analysis ◽  
The Past ◽  
History Of ◽  
Minimum Residence Time ◽  
Plant Macrofossil Analysis

We used the techniques of plant macrofossil analysis and radiocarbon dating to determine the minimum residence time of the endangered thread-leaved sundew (Drosera filiformis Raf.) in three bogs of southern Nova Scotia. The minimum residence times are 4240 cal. year BP for Swain's Road Bog, 2050 cal. year BP for Villagedale Bog, and very recent (modern radiocarbon age) for Port La Tour Bog, indicating that the species has likely been in the region for the past 4240 years. Analysis of nearby Old School Road Bog, which lacks D. filiformis today, failed to find any evidence that it formerly occurred there, suggesting that it may not have been more widespread in the past than it is today.Key words: Drosera filiformis, thread-leaved sundew, plant macrofossil analysis, radiocarbon dating, bogs, minimum residence time.

scholarly journals Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation

2016 ◽  
Vol 113 (13) ◽  
pp. 3465-3470 ◽  
Author(s):  
Thomas K. Bauska ◽  
Daniel Baggenstos ◽  
Edward J. Brook ◽  
Alan C. Mix ◽  
Shaun A. Marcott ◽  
...  
Keyword(s):  
Atmospheric Carbon Dioxide ◽  
Biological Pump ◽  
Ocean Temperature ◽  
Last Deglaciation ◽  
Surface Ocean ◽  
Stable Isotopic ◽  
Rapid Changes ◽  
The Last Deglaciation ◽  
Taylor Glacier ◽  
Equal Contribution

An understanding of the mechanisms that control CO2 change during glacial–interglacial cycles remains elusive. Here we help to constrain changing sources with a high-precision, high-resolution deglacial record of the stable isotopic composition of carbon in CO2 (δ13C-CO2) in air extracted from ice samples from Taylor Glacier, Antarctica. During the initial rise in atmospheric CO2 from 17.6 to 15.5 ka, these data demarcate a decrease in δ13C-CO2, likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric CO2 rise of 40 ppm is associated with small changes in δ13C-CO2, consistent with a nearly equal contribution from a further weakening of the biological pump and rising ocean temperature. These two trends, related to marine sources, are punctuated at 16.3 and 12.9 ka with abrupt, century-scale perturbations in δ13C-CO2 that suggest rapid oxidation of organic land carbon or enhanced air–sea gas exchange in the Southern Ocean. Additional century-scale increases in atmospheric CO2 coincident with increases in atmospheric CH4 and Northern Hemisphere temperature at the onset of the Bølling (14.6–14.3 ka) and Holocene (11.6–11.4 ka) intervals are associated with small changes in δ13C-CO2, suggesting a combination of sources that included rising surface ocean temperature.

Thermokarst Lakes as a Source of Atmospheric CH4 During the Last Deglaciation

Science ◽  
2007 ◽  
Vol 318 (5850) ◽  
pp. 633-636 ◽  
Author(s):  
K. M. Walter ◽  
M. E. Edwards ◽  
G. Grosse ◽  
S. A. Zimov ◽  
F. S. Chapin
Keyword(s):  
Organic Matter ◽  
Climate Warming ◽  
Methane Concentration ◽  
Ice Core ◽  
Atmospheric Methane ◽  
Last Deglaciation ◽  
Polar Ice ◽  
Thermokarst Lakes ◽  
The Last Deglaciation ◽  
Thaw Lakes

Polar ice-core records suggest that an arctic or boreal source was responsible for more than 30% of the large increase in global atmospheric methane (CH4) concentration during deglacial climate warming; however, specific sources of that CH4 are still debated. Here we present an estimate of past CH4 flux during deglaciation from bubbling from thermokarst (thaw) lakes. Based on high rates of CH4 bubbling from contemporary arctic thermokarst lakes, high CH4 production potentials of organic matter from Pleistocene-aged frozen sediments, and estimates of the changing extent of these deposits as thermokarst lakes developed during deglaciation, we find that CH4 bubbling from newly forming thermokarst lakes comprised 33 to 87% of the high-latitude increase in atmospheric methane concentration and, in turn, contributed to the climate warming at the Pleistocene-Holocene transition.

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