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.

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Sediments cored along the southwestern Iberian margin during Integrated Ocean Drilling Program Expedition 339 provide constraints on Mediterranean Outflow Water (MOW) circulation patterns from the Pliocene epoch to the present day. After the Strait of Gibraltar opened (5.33 million years ago), a limited volume of MOW entered the Atlantic. Depositional hiatuses indicate erosion by bottom currents related to higher volumes of MOW circulating into the North Atlantic, beginning in the late Pliocene. The hiatuses coincide with regional tectonic events and changes in global thermohaline circulation (THC). This suggests that MOW influenced Atlantic Meridional Overturning Circulation (AMOC), THC, and climatic shifts by contributing a component of warm, saline water to northern latitudes while in turn being influenced by plate tectonics.

Magnetic susceptibility of rocks can be dominated by diamagnetic and paramagnetic matrix minerals, ferrimagnetic and antiferromagnetic trace minerals, or a combination. The interpretation of magnetic fabric data (anisotropy of magnetic susceptibility, AMS) hinges on the qualitative and quantitative analysis of the sources of magnetic susceptibility. We discuss two methods that quantify the contribution of the different groups to the AMS: (1) comparative measurements of the magnetic susceptibility in low fields and high fields and (2) heating curves from 77 K to room temperature (low temperature magnetic susceptibility, LTMS). Method 1 measures paramagnetic, diamagnetic, and antiferromagnetic susceptibilities above the saturation magnetization of the ferrimagnetic minerals and method 2 interprets heating curves based on the fact that only the paramagnetic susceptibility is a function of temperature (Curie-Weiss law). Curie constants, paramagnetic Curie temperatures, and phase transitions (Verwey at 118 K: magnetite; Morin at 263 K: hematite) are diagnostic for specific minerals and provide further information about the contributing minerals of the sample. The relative contribution of the ferrimagnetic and paramagnetic minerals to the total susceptibility can be estimated from both methods with the same precision, if antiferromagnetic and diamagnetic contributions are insignificant. However, the LTMS method requires only simple equipment and procedures. The low temperature method can be extended to the three-dimensional case to decompose the total susceptibility tensor into its paramagnetic and ferrimagnetic sub-tensors (low temperature AMS, LTAMS). L1'MS and LTAMS are powerful additions to the group of magnetic fabric methods that allow the quantification of mineral preferred orientation in natural samples.

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Magnetic fabrics from rocks with multiple mineral-preferred orientations may have anisotropy ellipsoids whose shape and orientation arise from the addition of two or more component fabrics. Our numerical models and experiments demonstrate that such composite magnetic fabrics do not directly reflect the shapes and/or orientations of the individual mineral fabrics, and we provide criteria for the recognition and interpretation of composite fabrics in natural rocks. These criteria include: (1) the orientation of the maximum susceptibility axis is located at the intersection of two planar fabrics, and (2) the shape of the susceptibility ellipsoid changes from oblate to prolate and the degree of anisotropy decreases, as the relative intensity of two planar component fabrics becomes equal and as the angle between the planar fabrics increases.Composite magnetic fabrics are observed in the shales and slates of the Martinsburg Formation, Lehigh Gap, Pennsylvania. Modeling of the AMS (anisotropy of magnetic susceptibility) and ARMA (anhysteretic remanent magnetization anisotropy) behavior constrains the relative degree of anisotropy of the bedding-parallel and cleavage-parallel fabrics. In particular, ARMA model results allow a good estimate of magnetite fabric strength.We conclude that, in the presence of composite magnetic fabrics, quantitative measures of finite strain in deformed rocks are limited by the ability to accurately determine the degree of anisotropy and relative susceptibility of each component fabric. Such determinations require knowledge of the mineral(s) that are responsible for the measured magnetic fabric and their behavior during deformation.