A detailed record of eolian deposition from the Asian monsoon has been determined from measurements of magnetic susceptibility at Sites 721 and 722 located on an elevated ridge in the western Arabian Sea. Susceptibility was measured at 5-cm intervals (1 k.y.) on whole-core sections (15,000 measurements) and these data were used to construct complete composite records extending to 3.2 Ma. The composite records at Sites 721 and 722 are virtually identical, suggesting that both are complete and without local hiatuses. Bulk X-ray diffraction analyses indicate that (eolian) quartz and dolomite abundances are strongly covariant with the susceptibility data {r = 0.83, 0.70; « = 172). An almost perfect correlation (r = 0.98; n = 94) between susceptibility and terrigenous percent (determined by sequential carbonate, opal, and organic carbon extraction) indicates that susceptibility is a rapid and sensitive tracer of terrigenous fraction variations. Furthermore, terrigenous and biogenic flux calculations indicate that the observed susceptibility variations are indeed the result of terrigenous accumulation rate variations and not dilution by variations in biogenic input.In the frequency-domain, variance is concentrated at orbital periodicities for the entire 3.2 Ma sequence; however, there is a shift in the dominant periodicity at ca. 2.4 Ma. From 3.2 to 2.4 Ma, the susceptibility data vary almost purely at 23-19 k.y. periodicities and coherency with the precessional insolation forcing is high (0.89). Strong and coherent variance at the 23-19 k.y. band is maintained after 2.4 Ma but there is a dramatic increase in variance at the 41 k.y. periodicity corresponding to orbital obliquity. The timing of this shift coincides with the initiation of Northern Hemisphere glaciation and suggests a linkage between high-and low-latitude climate processes. General circulation model experiments and paleoclimatological evidence from northeast Africa suggest that the increase in 41 k.y. power after 2.4 Ma may be reflecting periodic increases in monsoon dust source area aridity due to the coeval expansion of the Eurasian and/or North American ice sheets, which varied predominantly at this periodicity.

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The Northwest Atlantic is a region of major climate change over the twentieth century, affected by the weakening of the Atlantic meridional overturning circulation. To assess whether the ability of this region to absorb anthropogenic CO 2 has been impacted by this change, we present the region's first long-term carbon isotope (δ 13 C) time series of fossil foraminifera spanning the past 4,000 years. These records reveal an unprecedented negative δ 13 C excursion driven by anthropogenic CO 2 penetration into the surface ocean, the "Suess effect" signal. This signal (amplitude −0.45‰) emerges in 1950 CE ± 15 with a decrease rate of 0.009 ± 0.001‰/yr. This marine signal is~30% of the atmospheric Suess effect and emerges over a century later. Based on current estimates of the ratio of δ 13 C DIC change to dissolved inorganic carbon change and limited constraints on surface ocean residence times, we calculate a mean anthropogenic CO 2 uptake rate of 0.6 ± 0.2 μmol/(kg yr) from 1950 to 2005. Plain Language SummarySince the industrial revolution, the burning of fossil fuels for human energy and transportation needs has caused an accumulation of carbon dioxide (CO 2 ) in the atmosphere. Over the same time period, nearly 30% of CO 2 emissions have been taken up by the ocean. This absorption is not uniform; therefore, understanding local CO 2 uptake rates is essential for assessing future ocean acidification risk. Our study investigates and presents the first long-term history of carbon for the Northwest Atlantic shelf region. The CO 2 emitted from fossil fuel burning has a distinct carbon isotope ratio compared to the preindustrial background level. Organisms called foraminifera incorporate the carbon isotope ratio of ocean carbon into their shells, which eventually sink to the seafloor where they are preserved in the sediments. For our analysis, we collected five sediment cores containing foraminifera from the NW Atlantic, resulting in carbon isotope records that span the last 4,000 years. We find evidence of fossil fuel-derived CO 2 in the NW Atlantic starting in 1950 and translate carbon isotope trends into estimates of fossil fuel CO 2 uptake rates by the surface ocean. Results from our study can be used to assess and predict future ocean acidification risk.

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