What was done
Working with a 2.5-m gravity core and an 18-cm box core taken from the outer area of the Ria de Muros (42°44'N, 9°02'W) on the northwestern coast of the Iberian Peninsula in June 2004, Andrade et al. established a climatic history of the region through "the combined use of textural analysis, magnetic properties and geochemical parameters (total concentrations of diagenetically stable and mobile elements in sediment and pore water)," which "allowed the identification of a current redox front and two palaeosedimentary redox fronts in the sediment record."

What was learned
The three redox fronts, as the team of Spanish scientists describes them, "originated during periods of high marine/terrestrial organic matter ratio (as inferred from the ratio of total organic carbon to total nitrogen and δ¹³C)." And they say that "sedimentation rates calculated from ¹⁴C dating results identify these periods as known periods of increased upwelling and reduced continental input due to colder, drier climate in the NW Iberian Peninsula, namely the Little Ice Age, the Dark Ages, and the first cold period of the Upper Holocene." They also add that the lesser proportion of oceanic influence observed between 1250 and 560 cal. yr BP) "coincides with the Medieval Warm Period, during which there was an increase in continental input to both the continental shelf (Mohamed et al., 2010) and the Rias of Vigo and Muros (Alvarez et al., 2005; Lebreiro et al., 2006)." And they add that the colder Dark Ages period was preceded by the "Roman Warm Period."

What it means
Once again, we find documentation for the millennial-scale oscillation of climate that has alternately brought the earth both into and out of the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period, the Little Ice Age, and -- most recently -- into the Current Warm Period. And during all of these climatic transitions, except for the most recent one, there have been no significant changes in the atmosphere's CO2 concentration, which suggests that the transition out of the Little Ice Age and into the Current Warm Period likely had nothing at all to do with the concomitant increase in the air's CO2 content.

References
Alvarez, M.C., Flores, J.A., Sierro, F.J., Diz, P., Frances, G., Pelejero, C. and Grimalt, J. 2005. Millennial surface water dynamics in the Ria de Vigo during the last 3000 years as revealed by coccoliths and molecular biomarkers. Palaeogeography, Palaeoclimatology, Palaeoecology 218: 1-13.

Lebreiro, S.M., Frances, G., Abrantes, F.F.G., Diz, P., Bartels-Jonsdottir, H.B., Stroynowski, Z.N., Gil, I.M., Pena, L.D., Rodrigues, T., Jones, P.D., Nombela, M.A., Alejo, I., Briffa, K.R., Harris, I. and Grimalt, J.O. 2006. Climate change and coastal hydrographic response along the Atlantic Iberian margin (Tagus Prodelta and Muros Ria) during the last two millennia. The Holocene 16: 1003-1015.

Mohamed, K.J., Rey, D., Rubio, B., Vilas, F. and Frederichs, T. 2010. Interplay between detrital and diagenetic processes since the last glacial maximum on the northwest Iberian continental shelf. Quaternary Research 73: 507-520.