Gideon M. Henderson

The sequence of events surrounding Termination II and their implications for the cause of glacial-interglacial CO2 changes

Events surrounding Termination II, as preserved in the Vostok ice core, provide a number of clues about the mechanisms controlling glacial to interglacial climate change. Antarctic temperature and the atmospheres CO2 content increased together over a period of ∼8000 years. This increase is bounded by a drop in dust flux at its onset and by a drop in the δ18O of trapped air at its finish. A similar lag between dust flux and foraminiferal δ18O is seen in a Southern Ocean marine record, suggesting that the δ18O in air trapped in Vostok ice is a valid proxy for ice volume. The synchronous change of atmospheric CO2 and southern hemisphere temperature thus preceded the melting of the northern hemisphere ice sheets. This observation, coupled with the fact that nutrient reorganization in the North Atlantic occurs with or after the sea level rise, eliminates many scenarios proposed to explain the CO2 rise, including those which rely on sea level change, conveyor-related nutrient redistribution, or North Atlantic cooling. Southern Ocean scenarios become the front runners, but the most popular mechanism, iron fertilization, has two problems in explaining the CO2 rise before Termination II. First, much of the dust demise occurs prior to the change in CO2, so if iron is the villain, a threshold value of its supply must be called upon above which productivity does not continue to increase. Second, the CO2 rise continues for some 4–5 kyr after the dust flux has fallen to close to zero. These problems may be solved if the increased iron supply in dust caused higher rates of nitrogen fixation during the glacial periods. In this case the residence time of oceanic nitrate of a few thousand years would enable decreasing productivity to be a global rather than a local phenomenon and would explain the slow rampup of atmospheric CO2.

Ice-sheet collapse and sea-level rise at the Bølling warming 14,600 years ago

Past sea-level records provide invaluable information about the response of ice sheets to climate forcing. Some such records suggest that the last deglaciation was punctuated by a dramatic period of sea-level rise, of about 20 metres, in less than 500 years. Controversy about the amplitude and timing of this meltwater pulse (MWP-1A) has, however, led to uncertainty about the source of the melt water and its temporal and causal relationships with the abrupt climate changes of the deglaciation. Here we show that MWP-1A started no earlier than 14,650 years ago and ended before 14,310 years ago, making it coeval with the Bølling warming. Our results, based on corals drilled offshore from Tahiti during Integrated Ocean Drilling Project Expedition 310, reveal that the increase in sea level at Tahiti was between 12 and 22 metres, with a most probable value between 14 and 18 metres, establishing a significant meltwater contribution from the Southern Hemisphere. This implies that the rate of eustatic sea-level rise exceeded 40 millimetres per year during MWP-1A.

Evidence from U-Th dating against Northern Hemisphere forcing of the penultimate deglaciation

Milankovitch proposed that summer insolation at mid-latitudes in the Northern Hemisphere directly causes the ice-age climate cycles. This would imply that times of ice-sheet collapse should correspond to peaks in Northern Hemisphere June insolation. But the penultimate deglaciation has proved controversial because June insolation peaks 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15 kyr earlier. There is a clear signature of the penultimate deglaciation in marine oxygen-isotope records. But dating this event, which is significantly before the 14C age range, has not been possible. Here we date the penultimate deglaciation in a record from the Bahamas using a new U-Th isochron technique. After the necessary corrections for α-recoil mobility of 234U and 230Th and a small age correction for sediment mixing, the midpoint age for the penultimate deglaciation is determined to be 135 ± 2.5 kyr ago. This age is consistent with some coral-based sea-level estimates, but it is difficult to reconcile with June Northern Hemisphere insolation as the trigger for the ice-age cycles. Potential alternative driving mechanisms for the ice-age cycles that are consistent with such an early date for the penultimate deglaciation are either the variability of the tropical ocean–atmosphere system or changes in atmospheric CO2 concentration controlled by a process in the Southern Hemisphere.

Lithium-isotope fractionation during continental weathering processes

A comprehensive understanding of lithium-isotope fractionation during terrestrial weathering is necessary in order to use lithium isotopes to trace chemical cycles, climatic changes and igneous processes. This study investigates lithium-isotope fractionation in two laboratory experiments and by analyses of natural basalt weathering products. Partial dissolution of basalts in the laboratory does not result in fractionation of lithium isotopes but similar dissolution of a granite sample causes significant fractionation. This may reflect dissolution of secondary minerals from the granite, or differences in the Li-isotope composition of primary minerals in this more evolved igneous rock. Significant Li-isotope fractionation was also observed during sorption onto mineral surfaces in the laboratory, although this was highly dependent on sample mineralogy. No fractionation accompanies the outer-sphere physisorption of Li to smectite surfaces. Some fractionation accompanies sorption onto ferrihydrite and significant fractionation with α=0.986 is seen during inner-sphere chemisorption to gibbsite surfaces. Repeat experiments with varying amounts of sorption demonstrate that Li remains in an exchangeable site on the gibbsite surface. The extent of fractionation onto gibbsite observed in this study (∼13‰) is about half that required in order for clay-surface removal to balance the ocean Li-isotope budget. Isotopic fractionation of Li was found to occur on a <300-yr timescale in both cold and warm natural environments. A minimally altered rock surface from Iceland was found to be two lighter in δ7Li than the sample interior, probably due to the preferential incorporation of 6Li in clay or oxide-rich alteration products on the sample surface. Soil samples from Hawaii also demonstrate Li fractionation during weathering. In this environment, rainwater (δ7Li=10) contributes a significant flux of isotopically heavy Li to the developing soil. Despite this, soils have similar Li-isotope compositions to the primary basalt (δ7Li=4), indicating that 6Li is preferentially retained during weathering. This conclusion is supported by isotopically heavy river water on Hawaii (δ7Li=22) and by the correlation between Li concentration and δ7Li in the soil. The lab- and field-based measurements of this study clearly demonstrate that lithium fractionates during weathering, potentially providing information about the weathering environment now and in the past.

The U-series Toolbox for Paleoceanography

The geochemistry of marine sediments is a major source of information about the past environment. Of the many measurements that provide such information, those of the U-series nuclides are unusual in that they inform us about the rate and timescales of processes. Oceanic processes such as sedimentation, productivity, and circulation, typically occur on timescales too short to be assessed using parent-daughter isotope systems such as Rb-Sr or Sm-Nd. So the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides (principally 14C) or the U-series nuclides. This makes the U-series nuclides powerful allies in the quest to understand the past ocean-climate system and has led to their widespread application over the last decade. As in other applications of the U-series, those in paleoceanography rely on fractionation of the nuclides away from secular equilibrium. In the oceanic setting, this fractionation is generally due to differences in the solubility of the various nuclides. The behavior of the U-series nuclides in the ocean environment was widely researched in the middle decades of the twentieth century. This work established knowledge of the concentrations of the nuclides in the various compartments of the ocean system, and of their fluxes between these compartments. Such understanding was comprehensively summarized in the Ivanovich and Harmon U-series volume (1992), particularly by Cochran (1992). Understanding of the behavior of the U-series nuclides has not advanced very dramatically in the decade since that summary but a major theme of research has been the use of this geochemical understanding to develop U-series tools to assess the past environment (Table 1⇓). View this table: Table 1. Summary of seawater data for U-series nuclides with paleoceanographic applications. S - soluble, I - insoluble. Full descriptions of the paleoceanographic uses and references are provided in the text. Further details of the half …

Introduction to U-series Geochemistry

During the last century, the Earth Sciences underwent two major revolutions in understanding. The first was the recognition of the great antiquity of the Earth and the second was the development of plate tectonic theory. These leaps in knowledge moved geology from its largely descriptive origins and established the modern, quantitative, Earth Sciences. For any science, and particularly for the Earth Sciences, time scales are of central importance. Until recently, however, the study of time scales in the Earth Sciences was largely restricted to the unraveling of the ancient history of our planet. For several decades, Earth scientists have used a variety of isotope chronometers to unravel the long-term evolution of the planet. A fuller understanding of the physical and chemical processes driving this evolution often remained elusive because such processes occur on time scales (1–105 years) which are simply not resolvable by most conventional chronometers. The U-series isotopes, however, do provide tools with sufficient time resolution to study these Earth processes. During the last decade, the Earth Sciences have become increasingly focused on fundamental processes and U-series geochemistry has witnessed a renaissance, with widespread application in disciplines as diverse as modern oceanography and igneous petrology. The uranium and thorium decay-series contain radioactive isotopes of many elements (in particular, U, Th, Pa, Ra and Rn). The varied geochemical properties of these elements cause nuclides within the chain to be fractionated in different geological environments. while the varied half-lives of the nuclides allows investigation of processes occurring on time scales from days to 105 years. U-series measurements have therefore revolutionized the Earth Sciences by offering some of the only quantitative constraints on time scales applicable to the physical processes that take place on the Earth. The application of U-series geochemistry to the Earth Sciences was thoroughly summarized in 1982 …

New oceanic proxies for paleoclimate

Environmental variables such as temperature and salinity cannot be directly measured for the past. Such variables do, however, influence the chemistry and biology of the marine sedimentary record in a measurable way. Reconstructing the past environment is therefore possible by ‘proxy’. Such proxy reconstruction uses chemical and biological observations to assess two aspects of Earth’s climate system – the physics of ocean–atmosphere circulation, and the chemistry of the carbon cycle. Early proxies made use of faunal assemblages, stable isotope fractionation of oxygen and carbon, and the degree of saturation of biogenically produced organic molecules. These well-established tools have been complemented by many new proxies. For reconstruction of the physical environment, these include proxies for ocean temperature (Mg/Ca, Sr/Ca, δ44Ca) and ocean circulation (Cd/Ca, radiogenic isotopes, 14C, sortable silt). For reconstruction of the carbon cycle, they include proxies for ocean productivity (231Pa/230Th, U concentration); nutrient utilization (Cd/Ca, δ15N, δ30Si); alkalinity (Ba/Ca); pH (δ11B); carbonate ion concentration (foraminiferal weight, Zn/Ca); and atmospheric CO2 (δ11B, δ13C). These proxies provide a better understanding of past climate, and allow climate–model sensitivity to be tested, thereby improving our ability to predict future climate change. Proxy research still faces challenges, however, as some environmental variables cannot be reconstructed and as the underlying chemistry and biology of most proxies is not well understood. Few proxies have been applied to pre-Pleistocene times – another challenge for future research. Only by solving such challenges will proxies provide a full understanding of the range of possible climate variability on Earth and of the mechanisms causing this variability.

Seawater ( 234 U/ 238 U) during the last 800 thousand years

Abstract Constraining the history of seawater (234U/238U) is important because this ratio is used to assess the validity of U/Th ages, and because it provides information about the past rate of physical weathering on the continents. This study makes use of U-rich slope sediments from the Bahamas in an attempt to reconstruct seawater (234U/238U) for the last 800 kyr. For the last 360 kyr, U/Th dating of these sediments provides ages and initial (234U/238U) values. Sixty-seven samples, largely from marine highstands, have initial (234U/238U) which scatter somewhat about the modern seawater value (∼1.145) but neither this scatter nor the average value increases with age of sample. These data contrast with published coral data and suggest that seawater (234U/238U) has remained within 15‰ of the modern value for the last 360 kyr. This confirms the rejection of coral U/Th ages where the initial (234U/238U) is significantly different from modern seawater. Data from older highstands, dated with δ18O stratigraphy or by the presence of the Brunhes/Matuyama (B/M) reversal at 780 kyr, allow seawater (234U/238U) to be assessed prior to the range of the 230Th chronometer. Unfortunately, diagenetic scatter in the data between the B/M reversal and 360 kyr is rather large, probably relating to low U concentrations for these samples. But there is no indication of a trend in seawater (234U/238U) with age. High U samples from close to the B/M reversal show less diagenetic scatter and an initial (234U/238U) that averages 1.102. This lower value can be explained by lower seawater (234U/238U) at the time of the B/M reversal, or by progressive loss of 234U from the sediment by α-recoil. A simple box model is presented to illustrate the response of seawater (234U/238U) to variations in riverine input, such as might be caused by changes in continental weathering. Comparison of the Bahamas (234U/238U) data with model results indicates that riverine (234U/238U) has not varied by more than 65‰ for any 100 kyr period during the last 360 kyr. It also indicates that the ratio of physical to chemical weathering on the continents has not been higher than at present for any extended period during the last 800 kyr.

Penultimate Deglacial Sea-Level Timing from Uranium/Thorium Dating of Tahitian Corals

Early Riser How glacial-interglacial cycles and the long-term variability of sea level depend on the amount of energy received by Earth from the Sun is unclear. Thomas et al. (p. 1186, published online 23 April; see the cover) report results from fossil corals found in Tahiti that indicate that sea level began to rise when insolation at 65° North latitude was near a minimum, not after it had begun to rise, as predicted by the Milankovitch theory. In contrast, the timing of the last deglaciation agrees well with the Milankovitch theory. Thus, glacial cycles do not behave as simply as the Milankovitch theory suggests. Sea levels rose during the penultimate deglaciation while Northern Hemisphere insolation was at a minimum. The timing of sea-level change provides important constraints on the mechanisms driving Earth’s climate between glacial and interglacial states. Fossil corals constrain the timing of past sea level by their suitability for dating and their growth position close to sea level. The coral-derived age for the last deglaciation is consistent with climate change forced by Northern Hemisphere summer insolation (NHI), but the timing of the penultimate deglaciation is more controversial. We found, by means of uranium/thorium dating of fossil corals, that sea level during the penultimate deglaciation had risen to ~85 meters below the present sea level by 137,000 years ago, and that it fluctuated on a millennial time scale during deglaciation. This indicates that the penultimate deglaciation occurred earlier with respect to NHI than the last deglacial, beginning when NHI was at a minimum.

U^Th dating of marine isotope stage 7 in Bahamas slope sediments

In order to understand the driving forces for Pleistocene climate change more fully we need to compare the timing of climate events with their possible forcing. In contrast to the last interglacial (marine isotope stage (MIS) 5) the timing of the penultimate interglacial (MIS 7) is poorly constrained. This study constrains its timing and structure by precise U^Th dating of high-resolution N 18 O records from aragonite-rich Bahamian slope sediments of ODP Leg 166 (Sites 1008 and 1009). The major glacial^interglacial cycles in N 18 O are distinct within these cores and some MIS 7 substages can be identified. These sediments are well suited for U^Th dating because they have uranium concentrations of up to 12 ppm and very low initial 230 Th contributions with most samples showing 230 Th/ 232 Th activity ratio of s 75. U and Th concentrations and isotope ratios were measured by thermal ionisation mass spectrometry and multiple collector inductively coupled plasma mass spectrometry, with the latter providing dramatically better precision. Twenty-nine of the 41 samples measured have a N 234 U value close to modern seawater suggesting that they have experienced little diagenesis. Ages from 27 of the 41 samples were deemed reliable on the basis of both their U and their Th isotope ratios. Ages generally increase with depth, although we see a repeated section of stratigraphy in one core. Extrapolation of constant sedimentation rate through each substage suggests that the peak of MIS 7e lasted from V237 to 228 ka and that 7c began at 215 ka. This timing is consistent with existing low precision radiometric dates from speleothem deposits. The beginning of both these substages appears to be slightly later than in orbitally tuned timescales. The end of MIS 7 is complex, but also appears to be somewhat later than is suggested by orbitally tuned timescales, although this event is not particularly well defined in these cores. fl 2002 Elsevier Science B.V. All rights reserved.