Hydrological variations of the intermediate water masses of the western Mediterranean Sea during the past 20?ka inferred from neodymium isotopic composition in foraminifera and cold-water corals

摘要

We present the neodymium isotopic composition (εNd) of mixed planktonic foraminifera species from a sediment core collected at 622m water depth in the Balearic Sea, as well as εNd of scleractinian cold-water corals (CWC; Madrepora oculata, Lophelia pertusa) retrieved between 280 and 442m water depth in the Alboran Sea and at 414m depth in the southern Sardinian continental margin. The aim is to constrain hydrological variations at intermediate depths in the western Mediterranean Sea during the last 20kyr. Planktonic (Globigerina bulloides) and benthic (Cibicidoides pachyderma) foraminifera from the Balearic Sea were also analyzed for stable oxygen (δ18O) and carbon (δ13C) isotopes. The foraminiferal and coral εNd values from the Balearic and Alboran seas are comparable over the last ?～?13kyr, with mean values of ?8.94±0.26 (1σ; n?=?24) and ?8.91±0.18 (1σ; n?=?25), respectively. Before 13kaBP, the foraminiferal εNd values are slightly lower (?9.28±0.15) and tend to reflect higher mixing between intermediate and deep waters, which are characterized by more unradiogenic εNd values. The slight εNd increase after 13kaBP is associated with a decoupling in the benthic foraminiferal δ13C composition between intermediate and deeper depths, which started at ?～?16kaBP. This suggests an earlier stratification of the water masses and a subsequent reduced contribution of unradiogenic εNd from deep waters. The CWC from the Sardinia Channel show a much larger scatter of εNd values, from ?8.66±0.30 to ?5.99±0.50, and a lower average (?7.31±0.73; n?=?19) compared to the CWC and foraminifera from the Alboran and Balearic seas, indicative of intermediate waters sourced from the Levantine basin. At the time of sapropel S1 deposition (10.2 to 6.4ka), the εNd values of the Sardinian CWC become more unradiogenic (?8.38±0.47; n?=?3 at ?～?8.7kaBP), suggesting a significant contribution of intermediate waters originated from the western basin. We propose that western Mediterranean intermediate waters replaced the Levantine Intermediate Water (LIW), and thus there was a strong reduction of the LIW during the mid-sapropel (?～?8.7kaBP). This observation supports a notable change of Mediterranean circulation pattern centered on sapropel S1 that needs further investigation to be confirmed.