Comparison of qualitative and quantitative dinoflagellate cyst approaches in reconstructing glacial-interglacial climate variability at West Iberian Margin IODP ‘Shackleton’ Site U1385

Abstract

A B S T R A C TDinoflagellate cysts (dinocysts) are commonly used to reconstruct past environmental conditions at the sea surface, such as primary production, temperature and salinity. Abundances of selected dinocyst taxa are used in qualitative indices, whereas the modern analogue technique (MAT) is used for quantitative reconstructions. Qualitative indices use process-based knowledge of present-day relations between the environmental variables and the distribution of dinocysts, whereas the MAT is based on the assumption that past assemblages have modern counterparts that correspond to similar sea-surface conditions. Here we explore the potential of both approaches to reconstruct sea surface temperature (SST), production (SSP), salinity (SSS) and seasonality during the last 22 thousand years along the West Iberian Margin (WIM), at Integrated Ocean Drilling Program (IODP) Site U1385. We compare results to published paleoclimatic reconstructions. Our SST and SSS reconstructions provide the first continuous dinocyst MAT-based SST and SSS records for this area and time interval.Qualitative and quantitative dinocyst-based SST estimates from the WIM are similar and resemble previous SST estimates from dinocysts, alkenones and foraminifers from nearby sites. The surface temperature trends and millennial-scale variations largely match those from the Greenland ice core records. Quantitative MAT-based SST estimates show increased seasonality in the glacial stage resulting from strong winter cooling. Dinocyst MAT-based salinity decreases concomitantly with cooling during the Younger Dryas and Heinrich Stadial 1 (HS1), likely related to the melting of icebergs that reached the region during HS1. Our qualitative and quantitative SSP estimates show higher values in the glacial stage compared to the Holocene, which is consistent with published records and supports the usefulness of both approaches. Small differences between SST tracer records may be explained by the limited number of modern analogues from warm ocean regions in the dinocyst reference dataset for MAT, the small number of dinocysts used in the qualitative estimates, the possible effect of a parameter other than temperature that might amplify noise, and/or seasonal biases of the tracer species.In any case, the advantage of the qualitative approach is to allow reconstructions in non-analogue situations. Regression of the qualitative index versus present-day SST also allows for a quantitative reconstruction, although over a limited range of SSTs (especially towards the lower end) and with quite some uncertainty, but produces reasonable values beyond the upper limit of the MAT. The MAT reconstructs more reliable SSTs, with a much smaller error of prediction, but only up to present-day WIM values, because it is based on the assumption that past assemblages have modern counterparts that correspond to similar sea-surface conditions, which is not always valid. The advantages of the MAT approach include its quantitative nature and insights into seasonality. Hence both app...

Towards quantitative environmental reconstructions from ancient non-analogue microfossil assemblages: Ecological preferences of Paleocene – Eocene dinoflagellates

“…Temperature dependent distribution is well known for many biota and species of dinocysts have been successfully employed as relative temperature indicators in a number of sections both in recent and deep time sediments (e.g. Brinkhuis et al, 1998;Penaud et al, 2010;Datema et al, 2017). Indeed, a number of dinocyst groups in our data appear temperature limited (Fig.…”

“…Indeed, many species have distinct environmental affinities in the modern ocean (Zonneveld et al, 2013). Modern analog techniques based on core-top sediment assemblages and environmental parameters have been successfully applied to the recent past (De Vernal et al, 2005;Radi and de Vernal, 2008;Datema et al, 2017). For deep time, however, we have to rely on the relationship of dinocyst species, genera or families to independent environmental proxy data.…”

Millennial‐Scale Climate Variability and Dinoflagellate‐Cyst‐Based Seasonality Changes Over the Last ~150 kyrs at “Shackleton Site” U1385

“…In the western North Atlantic, the Gulf Stream branches off into the North Atlantic Current toward the northeast and the Azores Current toward the southeast. The zone between the North Atlantic Current and the Azores Current is marked by weak circulation supplying the large‐scale Portugal Current System that represents the surface currents along the WIM (Pérez et al, ; Figure 2 in Datema et al, ). The WIM has a narrow continental shelf with a steep upper slope that is intersected by promontories and indented by canyons (Figure ).…”

“…This is not surprising in view of the interdependency of winter and summer temperature and uniform seasonal gradients in the reference data set. For the West Iberian Margin (WIM), Datema et al () used dinoflagellate cysts (dinocysts) to reconstruct summer and winter SSTs at the scale of the last 22 kyrs, indicating stronger cooling in winter than in summer during the LGM. While results are promising, this approach has not yet been widely applied, spatially and/or temporally (Carré & Cheddadi, ).…”

“…In this study we constructed a millennial‐scale seasonality record for the WIM using dinocyst summer and winter MAT‐based SSTs, extending the Datema et al () record to cover the full last glacial cycle. We use sediments deposited over the last 152 kyr at the Integrated Ocean Drilling Program (IODP) Site U1385 (Hodell, Crowhurst, et al, ) along the WIM.…”