Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.
The post-middle Miocene evolution of sedimentary patterns in the eastern equatorial Pacific Ocean has been deduced from a compilation and synthesis of CaCO 3 , opal, and nannofossil assemblage data from 11 sites drilled during Leg 138. Improvements in stratigraphic correlation and time scale development enabled the construction of lithostratigraphic and chronostratigraphic frameworks of exceptional quality. These frameworks, and the high sedimentation rates (often exceeding 4 cm/k.y.) provided a detailed and synoptic paleoceanographic view of a large and highly productive region. The three highlights that emerge are: (1) a middle late Miocene "carbonate crash" (Lyle et al., this volume); (2) a late Miocene-early Pliocene "biogenic bloom"; and (3) an early Pliocene "opal shift". During the carbonate crash, an interval of dissolution extending from-11.2 to 7.5 Ma, CaCO 3 accumulation rates declined to near zero over much of the eastern equatorial Pacific, whereas opal accumulation rates remained substantially unchanged. The crash nadir, near 9.5 Ma, was marked by a brief shoaling of the regional carbonate compensation depth by more than 1400 m. The carbonate crash has been correlated over the entire tropical Pacific Ocean, and has been attributed to tectonically-induced changes in abyssal flow through the Panamanian seaway. The biogenic bloom extended from 6.7 to 4.5 Ma, and was characterized by an overall increase in biogenic accumulation and by a steepening of the latitudinal accumulation gradient toward the equator. The bloom has been observed over a large portion of the global ocean and has been linked to increased productivity. The final highlight, is a distinct and permanent shift in the locus of maximum opal mass accumulation rate at 4.4 Ma. This shift was temporally, and perhaps causally, linked to the final closure of the Panamanian seaway. Before 4.4 Ma, opal accumulation was greatest in the eastern equatorial Pacific Basin (near 0°N, 107°W). Since then, the highest opal fluxes in the equatorial Pacific have occurred in the Galapagos region (near 3°S, 92°W).
Core V21‐146 provides a continuous record of northwest Pacific pelagic sedimentation spanning the past 530,000 years. Downcore variations of δ18O from benthic foraminiferal calcite were correlated to the SPECMAP record to provide an age model for late Pleistocene paleoclimatic and paleoceanographic variation. Fluxes of CaCO3 and eolian material have been determined by combining information about linear sedimentation rates, bulk densities and component percentages. The mass accumulation rate (MAR) of CaCO3 varies from 58 to 1435 mg(cm² kyr)−1 and exhibits the Pacific Ocean pattern of higher accumulation associated with glacial periods for the younger portion of the record. However, the pattern of CaCO3 accumulation appears to shift during the mid‐Brunhes so that prior to 350,000 years ago it exhibits an opposite pattern of greater accumulation during interglacial periods. The flux of eolian dust is a proxy measure of the source area aridity and correlates well to the loess‐soil stratigraphy in China. Dust fluxes range from 43 to 718 mg(cm² kyr)−1 and are greater by a factor of about 4 during glacial times. Cross‐spectral analysis of the eolian flux and oxygen isotope records shows an inphase and coherent relationship at each of the major orbital periodicities of 100, 41, and 19 kyr. Eolian grain sizes, generally used as a proxy record of wind intensity, range from 8.4ф (2.9 µm) to 6.2ф (13.6 µm). A change in grain size variability occurs near about 300 ka such that the older portion is characterized by lower‐frequency and higher‐amplitude fluctuations than the younger portion. Power spectra for the eolian grain size record show dominant peaks at 100 and 50 kyr and a broad 25–35 kyr peak but coherency to the oxygen isotope curve at only the 100 and 33 kyr periodicities. The phase relationship for eccentricity between the eolian grain size and δ18O records suggests that at the 100 kyr frequency the smallest grain sizes are associated with glacial conditions.
Abstract. Mineral dust plays an important role in the climate system by interacting with radiation, clouds, and biogeochemical cycles. In addition, natural archives show that the dust cycle experienced variability in the past in response to global and local climate change. The compilation of the DIRTMAP (Dust Indicators and Records from Terrestrial and MArine Palaeoenvironments) paleodust data sets in the last 2 decades provided a benchmark for paleoclimate models that include the dust cycle, following a time slice approach. We propose an innovative framework to organize a paleodust data set that builds on the positive experience of DIRTMAP and takes into account new scientific challenges by providing a concise and accessible data set of temporally resolved records of dust mass accumulation rates and particle grain size distributions. We consider data from ice cores, marine sediments, loess–paleosol sequences, lake sediments, and peat bogs for this compilation, with a temporal focus on the Holocene period. This global compilation allows the investigation of the potential, uncertainties, and confidence level of dust mass accumulation rate reconstructions and highlights the importance of dust particle size information for accurate and quantitative reconstructions of the dust cycle. After applying criteria that help to establish that the data considered represent changes in dust deposition, 45 paleodust records have been identified, with the highest density of dust deposition data occurring in the North Atlantic region. Although the temporal evolution of dust in the North Atlantic appears consistent across several cores and suggests that minimum dust fluxes are likely observed during the early to mid-Holocene period (6000–8000 years ago), the magnitude of dust fluxes in these observations is not fully consistent, suggesting that more work needs to be done to synthesize data sets for the Holocene. Based on the data compilation, we used the Community Earth System Model to estimate the mass balance of and variability in the global dust cycle during the Holocene, with dust loads ranging from 17.2 to 20.8 Tg between 2000 and 10 000 years ago and with a minimum in the early to mid-Holocene (6000–8000 years ago).
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