摘要:

Air‐sea exchange represents an important controlling factor for the distribution of carbon isotopes in the modern ocean, even below the surface mixed layer. Here we show that this thermodynamic influence varies between water masses with a magnitude which is large relative to the amplitude of typical deep sea sediment δ13C records. Thus, changes in air‐sea exchange processes must be regarded as a significant source of variability for any marine δ13C s

ISSN:0883-8305    

DOI:10.1029/93PA01803

年代:1993

数据来源: WILEY

摘要:

Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000‐year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000‐year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000‐year cycle requires a source of climatic inertia having a time constant (∼15,000 years) much larger than the other cycles (∼5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity‐driven modulations in the 23,000‐year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher‐frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000‐year oscillations.Whether a glacier or ice sheet influences the climate depends very much on the scale…. The interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger, thereby gradually increasing its radius of influence.Johannes Oerl

ISSN:0883-8305    

DOI:10.1029/93PA02751

年代:1993

数据来源: WILEY

摘要:

The object of the present study is to introduce a means of comparing the Vostok and marine chronologies. Our strategy has been to use the δ18O of atmospheric O2(denoted δ18Oatm) from the Vostok ice core as a proxy for the δ18O of seawater (denoted δ18Osw). Our underlying premise in using δ18Oatmas a proxy for δ18Oswis that past variations in δ18Osw(an indicator of continental ice volume) have been transmitted to the atmospheric O2reservoir by photosynthesizing organisms in the surface waters of the world's oceans. We compare our record of δ18Oatmto the δ18Oswrecord which has been developed from studies of the isotopic composition of biogenic calcite (δ18Oforam) in deep‐sea cores. We have tied our δ18Oatmrecord from Vostok to the SPECMAP timescale throughout the last 135 kyr by correlating δ18Oatmwith a δ18Oswrecord from V19‐30. Results of the correlation indicate that 77% of the variance is shared between these two records. We observed differences between the δ18Oatmand the δ18Oswrecords during the coldest periods, which indicate that there have been subtle changes in the factors which regulate δ18Oatmother than δ18Osw. Our use of δ18Oatmas a proxy for δ18Oswmust therefore be considered tentative, especially during these periods. By correlating δ18Oatmwith δ18Osw, we provide a common temporal framework for comparing phase relationships between atmospheric records (from ice cores) and oceanographic records constructed from deep‐sea cores. Our correlated age‐depth relation for the Vostok core should not be considered an absolute Vostok timescale. We consider it to be the preferred timescale for comparing Vostok climate records with marine climate records which have been placed on the SPECMAP timescale. We have examined the fidelity of this common temporal framework by comparing sea surface temperature (SST) records from sediment cores with an Antarctic temperature record from the Vostok ice core. We have demonstrated that when the southern ocean SST and Antarctic temperature records are compared on this common temporal framework, they show a high degree of similarity. We interpret this result as supporting our use of the common temporal framework for comparing other climate records from the Vostok ice core with any climate record that has been correlated

ISSN:0883-8305    

DOI:10.1029/93PA02328

年代:1993

数据来源: WILEY

摘要:

If Heinrich events result from free oscillations in the size and basal melting conditions of the Laurentide ice sheet, a rigorous quantitative analysis of ice sheet physics should describe their dynamics. To explore this possibility, I exploit two characteristic timescales which arise from ice sheet physics to construct a relaxation oscillator model of the North Atlantic's Heinrich events. The numerical implementation of this model confirms the notion that the periodicity of Heinrich events (approximately 7,000 years) is determined by the gross properties of a steady glacial climate (e.g., an annual average sea level temperature of −10° C and an adiabatic lapse rat

ISSN:0883-8305    

DOI:10.1029/93PA02201

年代:1993

数据来源: WILEY

摘要:

Ice‐rafted debris in sediment cores from the North Atlantic suggests that the Laurentide ice sheet (LIS) periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7,000 years. Here, I propose that Heinrich events (i.e., what these episodes are called) were caused by free oscillations in the flow of the Laurentide ice sheet which arose because the floor of Hudson Bay and Hudson Strait is covered with soft, unconsolidated sediment that forms a slippery lubricant when thawed. The proposed Heinrich event cycle has two phases. The growth phase occurs when the sediment is frozen and the LIS is stranded (immobile) on a rigid bed. The volume of the LIS slowly grows during this phase at a rate dictated by snow accumulation. The purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway (i.e., an ice stream such as those which occur in West Antarctica today) developes through Hudson Strait. The volume of the LIS rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the Labrador Sea. The periodicityT=π/κ(−kθsl2G∼)2≈7000years of the proposed Heinrich event cycle is a function of the thermal conductivity and diffusivity of ice,kand κ, respectively, the atmospheric sea level temperature θsl(in degrees Celsius), and the excess geothermal heat flux defined byG∼=G−kΓwhere Γ is the atmospheric lapse rate, andGis the geothermal heat flux. Agreement between the predictedTand the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism I propose. An alternative mechanism in which Heinrich events are forced by periodic variations in external climate is implausible, because periodic atmospheric temperature perturbations are strongly attenuated with dep

ISSN:0883-8305    

DOI:10.1029/93PA02200

年代:1993

数据来源: WILEY

摘要:

A series of general circulation model experiments using Global Environmental and Ecological Simulation of Interactive Systems (GENESIS) were executed to evaluate the sensitivity of simulated mid‐Cretaceous climate to small perturbations in ocean heat transport. Three experiments were performed: (1) mixed layer ocean with no ocean heat transport, ZEROQ, (2) ocean heat transport specified as required for GENESIS to best match modern observations, ONEQ, and (3) doubled ocean heat transport, TWOQ. The ONEQ experiment represents an ocean heat transport which is actually about 15% of the values given by Carissimo et al. (1985) from modern observations. As a sensitivity experiment these model simulations represent a doubling of the role of the ocean. However, relative to the observations, they represent small perturbations to the total poleward heat transport in the model. With the exception of the tropics, no major changes in the structure of the general circulation of the atmosphere resulted from the modification of the ocean heat transport. However, relatively small increases in ocean heat transport resulted in a number of significant differences between simulations, including tropical cooling, polar warming, weakened equator‐to‐pole surface temperature gradients, weakened midlatitude jets, decreased land‐sea pressure contrast, and decreased midlatitude storminess. The experiments indicate that changes in ocean heat transport which are well within the realm of possibility for Earth history can have significant climatic impact. Although ocean heat transport may be a significant factor in explaining Cretaceous polar warmth, the changes specified in these experiments alone are not sufficient to explain the polar warmth of the Cre

ISSN:0883-8305    

DOI:10.1029/93PA02227

年代:1993

数据来源: WILEY

摘要:

A highly simplified coastal ocean model is constructed for central California during the middle to late Miocene (13–6 million years ago). Central California was a semienclosed shallow basin during that time, communicating with the open North Pacific Ocean. Surface temperatures for the basin have been derived using Tertiary fossil mollusks and siliceous deposits. The model provides qualitative information for reconstructing paleoclimatic and paleoceanic conditions. It is based on spatially two‐dimensional equations for the velocity and temperature in a single layer of shallow fluid. The model domain consists of a rectangular coastal basin partially connected to a portion of the Pacific Ocean. The wind stress is assumed to be spatially constant from a given direction. The flow in and out of the basin is driven by the bottom topography torque due to depth variation between basin and ocean. The temperature of the open ocean is prescribed. The temperature of the basin is governed by the relative influence of this oceanic temperature and the atmospheric reference temperature, to which vertical fluxes drive the basin in the absence of dynamical influences. Two different boundaries are used: fully open or semiopen, with a partial barrier between the basin and the ocean. We examine scenarios with different barrier extents, with openings to the ocean at one or both ends of the basin, and with different assumptions about the wind direction and currents. The effect of adding a second model layer is also investigated. We found that the results are quite insensitive to assumptions about the paleowind stress. The semiopen model appears closest to the paleocean data. This argues for the presence of a barrier during the late Miocene between the central California basin and the open ocean, at least in the basin's northern p

ISSN:0883-8305    

DOI:10.1029/93PA02336

年代:1993

数据来源: WILEY

摘要:

High‐resolution stable isotopic records are presented for the epi‐benthic foraminiferCibicidoides, the inferred shallow‐dwelling planktonicGlobigerinoides quadrilobatus, and the inferred deep‐dwelling planktonicGloboquadrina dehiscensfrom the middle Miocene (∼16–12 Ma) of Deep Sea Drilling Project site 588A, Lord Howe Rise, southwest Pacific. High‐resolution, multiple species oxygen and carbon isotopic data define the timing and character of the well‐known middle Miocene climatic‐oceanographic transition with a resolution comparable to Quaternary records. The benthic foraminiferal δ18O record is marked by several large fluctuations from ∼16 to 14.8 Ma, followed by a series of rapid (<50 kyr) δ18O increases that suggest a new state of the ocean‐climate system after 14.8 Ma. The total middle Miocene benthic oxygen isotopic increase of 1.2‰ is largely incorporated in two steps, an increase of 0.8‰ from 14.5 to 14.0 Ma and a second increase of 0.7‰ from 13.45 to 12.45 Ma. Each step is comprised of a series of marked δ18O increases, indicative of rapid East Antarctic ice sheet growth and contemporaneous deepwater cooling. A strong covariance of 0.7‰ between the benthic and deep‐dwelling planktonic species from 14.5 to 14.0 Ma (including a rapid increase from 14.1 to 14.05 Ma) suggests a 0.7‰ increase in the δ18O composition of seawater (δ18Osw) because of East Antarctic ice sheet growth. Comparison of the δ18O record ofGs. quadrilobatussuggests that surface waters warmed at this site by ∼3°C from 14.1 to 13.6 Ma. Carbon isotopic time series for each species generally covary throughout the early to middle Miocene interval (∼16–12 Ma), confirming that δ13C variations in this interval largely represent reservoir changes. High‐resolution δ13C data allow improved resolution of the latter five of six δ13C maxima within the well‐known early to middle Miocene carbon isotopic excursion (the Monterey Carbon Isotopic Excursion from 17.0 to 13.5 Ma). This is useful for global correlation. The last of these maxima ends with a 1‰ decrease centered from 13.9 to 13.7 Ma, ∼300 kyr after the δ18O increase considered to reflect East Antarctic ice growth. Covariance between benthic δ18O and δ13C from ∼16 to 13.8 Ma suggests a sensitive relation between global carbon cycling and the ocean‐climate system prior to 13.8 Ma. Episodic increases in organic carbon burial may have contributed to deep‐sea benthic δ13C maxima and synchronous global cooling. The positive relationship ended at ∼13.8 Ma, indicative of changing relations between global carbon cycling and the ocean‐climate system brought on by the increased stability of the E

ISSN:0883-8305    

DOI:10.1029/93PA02196

年代:1993

数据来源: WILEY

摘要:

The use of skeletal oxygen isotopic records for use in paleotemperature reconstruction has been hampered by the lack of independent evidence for ocean water oxygen isotopic composition. The δ18O record from homeothermic cetaceans has provided an independent estimate of ocean δ18O values represented by the Calvert and Choptank formations of Maryland. Fish teeth and bones (especially shark and ray teeth) were also collected from these sediments and provide the basis for paleotemperature estimates for represented time slices of the middle Miocene. Trends in δ18O values of the fish phosphate throughout the Calvert Formation are of opposite polarity to the trends from the cetacean bone phosphate. Paleotemperatures calculated using the cetacean proxies for ocean δ18O values sharpen the already present trend, indicating that ocean temperatures increased during episodes of greater glaciation and decreased during periods of lesser or no glaciation. When using modern average ocean values of 0‰ SMOW in the paleotemperature calculation, however, corrected paleotemperatures for the Choptank Formation do not alter the existing pattern of tempera

ISSN:0883-8305    

DOI:10.1029/93PA01412

年代:1993

数据来源: WILEY

摘要:

The white Chalk exposed in quarries at Lägerdorf and Kronsmoor, northwestern Germany, provides a standard section for the European Upper Cretaceous. The87Sr/86Sr values of nannofossil chalk and belemnite calcite increase upward through 330 m of section, from ≤0.70746 in the Upper Santonian to ≥0.70777 in the Lower Maastrichtian. The data define three linear trends separated by major points of inflection at stratigraphic heights in the section of 162 m (75.5 Ma) in the Upper CampanianGalerites vulgariszone and at −6 m (82.9 Ma), just above the base of the Campanian in theInoceramus lingua/Gonioteuthis quadratazone. The temporal rate of change of87Sr/86Sr was constant through each of the linear segments of our isotope “curve” when viewed at the resolution of our average sampling interval (0.15 m.y.). Fine structure, if real, may record brief (<100 kyr) excursions of87Sr/86Sr from values expected from the overall trends. In Lägerdorf, the boundary between the Santonian and Campanian stages, taken here as the level of first occurrence of the belemniteGonioteuthis granulataquadrata, has an87Sr/86Sr of 0.707473±5. This is within error of the values of 0.707457±16 for this boundary in the U.S. western interior (base of theScaphites leeiIII zone) and 0.707479±9 for this boundary in the English Chalk (top of theMarsupites testudinariuszone). In Kronsmoor, the boundary between the Campanian and Maastrichtian stages, taken here as the level of first occurrence of the belemniteBelemnella lanceolata, has an87Sr/86Sr of 0.707723±4. This is within error of the values of 0.707725±20 for this boundary in the U.S. western interior (base of theBaculites eliasizone) and 0.707728±5 for this boundary in the English Chalk (defin

ISSN:0883-8305    

DOI:10.1029/93PA02324

年代:1993

数据来源: WILEY