Abstract
Abstract. We evaluate the contribution of natural variability to the modern decrease in foraminiferal δ18O by relying on a 2200-yr-long, high-resolution record of oxygen isotopic ratio from a Central Mediterranean sediment core. Pre-industrial values are used to train and test two sets of algorithms that are able to forecast the natural variability in δ18O over the last 150 yr. These algorithms are based on autoregressive models and neural networks, respectively; they are applied separately to each of the δ18O series' significant variability components, rather than to the complete series. The separate components are extracted by singular-spectrum analysis and have narrow-band spectral content, which reduces the forecast error. By comparing the sum of the predicted low-frequency components to its actual values during the Industrial Era, we deduce that the natural contribution to these components of the modern δ18O variation decreased gradually, until it reached roughly 40%, as early as the end of the 1970s.
Highlights
Many different archives have been analyzed in the Mediterranean area for the reconstruction of the main physical and chemical parameters characterizing climate over the last millennia
This record was obtained from a shallow-water sediment core drilled in the Central Mediterranean (Gallipoli Terrace in the Gulf of Taranto, Ionian Sea) and was dated with high accuracy by tephroanalysis and radiometric measurements
We address the problem of evaluating the contribution of natural climate oscillations to this modern variation; pre-industrial δ18O variations are used to design and tune algorithms able to forecast the natural variability in the δ18O series over the last 150 yr
Summary
Many different archives have been analyzed in the Mediterranean area for the reconstruction of the main physical and chemical parameters characterizing climate over the last millennia. In a previous paper (Taricco et al, 2009), we presented a high-resolution record of foraminiferal δ18O isotopic ratio that covers the last two millennia. This record was obtained from a shallow-water sediment core drilled in the Central Mediterranean (Gallipoli Terrace in the Gulf of Taranto, Ionian Sea) and was dated with high accuracy by tephroanalysis and radiometric measurements. The δ18O series spans the last 2200 yr and was analyzed by singular-spectrum analysis (SSA; see Vautard and Ghil, 1989; Ghil and Vautard, 1991; Plaut et al, 1995; Ghil and Taricco, 1997; Ghil et al, 2002 and references therein) This analysis revealed highly significant decadal, centennial and multicentennial oscillatory components, along with a millennial trend.
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