Unexpectedly low organic matter content in Cariaco Basin sediments during the Younger Dryas: Origin and implications

TLDR: The Cariaco Basin (offshore Venezuela), the second largest anoxic basin after the Black Sea, knew intense upwelling activity and maximum primary production during the Younger Dryas (12.9-11.5 ka cal. BP) as discussed by the authors.

About: This article is published in Comptes Rendus Geoscience.The article was published on 2011-05-01 and is currently open access. It has received 9 citations till now. The article focuses on the topics: Younger Dryas & Total organic carbon.

Figures

Fig. 5. A. MAR of the different sediment components (L. Peterson, Pers. Comm., 2009). B. Computed Corg MAR of the different sources of organic carbon. C. Computed fraction of the TOC for the different sources of organic carbon. Fig. 5. A. Taux d’accumulation massique (MAR) des différentes fractions du sédiment (L. Peterson, Comm. Pers., 2009). B. Corg MAR calculé pour les différentes sources de carbone organique. C. Fraction du TOC calculée pour les différentes sources de carbone organique.

Fig. 2. Temporal variations of Cariaco sediment color and geochemical parameters. Sediment color from Hughen et al. (1996b) (core PL07-56PC). Squares: this study; diamonds: Piper and Dean, 2002 and L. Peterson Pers. Comm., 2009; triangles: Aycard (2004). PB: Preboreal, YD: Younger Dryas, BA: Bølling Allerød. Fig. 2. Variations temporelles de la couleur des sédiments de Cariaco et des paramètres géochimiques. Couleur du sédiment d’après Hughen et al. (1996b) (carotte PL07-56PC). Carrés : cette étude; losanges : Piper and Dean, 2002 et L. Peterson Comm. Pers., 2009 ; triangles : Aycard (2004). PB : Préboréal, YD : Dryas récent, BA : Bølling Allerød.

Fig. 3. Cross plot of the total organic carbon content (TOC) and the different mineral fractions of Cariaco sediments. Data only covering the BA to PB interval. Fig. 3. Graphiques de corrélation du contenu en carbone organique (COT) et des différentes fractions minérales du sédiment de Cariaco. Les données ne montrent que la période allant du BA au PB.

Fig. 1. Location map of the Cariaco Basin, studied cores and sediment trap discussed in the text. The grey area corresponds to present-day average position of the upwelling cell. Fig. 1. Carte de localisation du bassin de Cariaco, des carottes étudiées et des pièges à sédiments discutés dans le texte. La zone en gris correspond à la position moyenne actuelle de la cellule d’upwelling. Adapted from Müller-Karger et al., 2010.

Fig. 4. A. Cross plot of organic carbon MAR versus MAR of the mineral fraction of Cariaco sediments. B. Same data without CaCO3 MAR. Data from core PL07-39PC covering the totality of the last 14,7 ka (L. Peterson, Pers. Comm., 2009). 810m sediment trap data from Thunell et al. (2007). Fig. 4. A. Taux d’accumulation massique (MAR) du carbone organique en fonction de celui de la fraction minérale. B. Mêmes données sans les carbonates. Les données proviennent de la carotte PL07-39PC et couvrent la totalité des derniers 14,7 ka (L. Peterson, Comm. Pers., 2009). Piège à sédiment à 810m d’après Thunell et al. (2007).

Frequently asked questions

Q1. What are the contributions in "Unexpectedly low organic matter content in cariaco basin sediments during the younger dryas: origin and implications origine et implications de la très faible teneur en matière organique dans les sédiments du bassin de cariaco au dryas récent" ?

The present study demonstrates that the low TOC values result primarily from a relatively reduced flux of organic matter ( OM ) to the sediment and minor dilution by carbonate. 

Q2. What is the role of organic-walled organisms in the formation of future petroleum sources?

While diatoms are important to quantitatively export organic carbon and have an important role in the carbon cycle (Ragueneau et al., 2000), organic-walled organisms appear determinant in the formation of (very) organic-rich sediments and therefore in the formation of future petroleum source rocks. 

Q3. What is the first explanation to the reduction of the flux of OM relative to the mineral flux?

A first explanation to the reduction of the flux of OM relatively to the mineral fluxduring the YD is that OM was more efficiently degraded, either in the water column or within the sediment. 

Q4. What is the reason for the lower carbonate content of the sediment during the YD?

The higher CaCO3 content of the sediment during the YD therefore reflects a lower dissolution of carbonates and more alkaline conditions in the water column. 

Q5. What is the reason for the low TOC values observed during the YD?

Despite very good preservation, the concentration and MAR of dinoflagellate cysts markedly decrease during the YD (Mertens et al., 2009b). 

Q6. What is the reason why the Cariaco upwelling cell was smaller during the BA?

Recent modeling (Lane-Serff and Pearce, 2009) showed that due to lower sea level, the Cariaco upwelling cell was smaller and shifted to the east during the BA, so that ODP Site 1002 was relatively distant from the upwelling center compared to the sites of core PL07-39PC and MD03-2625 (Fig. 1). 

Q7. What is the main reason for the low TOC values during the YD?

From these observations, the carbonate content, that varies antithetically to TOC content through the BAYD- PB intervals (Fig. 2), appears as a diluting phase for the OM during the YD. 

Q8. What is the reason for the decreased Corg/mineral flux ratio?

From these observations, the decreased Corg/mineral flux ratio observed during theYD interval can be explained by a relative decrease of the contribution from organic walled organisms compared to diatoms. 

Q9. What is the main cause of the low TOC values observed during the YD?

From this observation, it appears that if carbonate dilution contributes to the low TOC values observed during the YD, it is not the major cause. 

Q10. What is the effect of mineral ballasting on the OM flux?

Modern trap studies show that mineral ballasting has a strong effect on the OM flux tothe sediment in the Cariaco Basin, such that the TOC and total mineral fluxes in the water column are strongly correlated (Thunell et al., 2007; Fig. 4A). 

Q11. What is the contribution of organic-walled organisms to total OM in the Caria?

The present study of the sediments of the Cariaco Basin however shows that the contribution of OM derived from organic-walled organisms is far from negligible and could represent more than 50% of the OM, despite abundant presence of diatoms in surface waters.