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Re-Evaluation of the Age Model for North Atlantic Ocean Site 982 – Arguments for a Return to the Original Chronology

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Re-Evaluation of the Age Model for North Atlantic Ocean Site 982 – Arguments for a Return to the Original Chronology Clim. Past, 9, 2391–2397, 2013 Open Access www.clim-past.net/9/2391/2013/ Climate doi:10.5194/cp-9-2391-2013 © Author(s) 2013. CC Attribution 3.0 License. of the Past Re-evaluation of the age model for North Atlantic Ocean Site 982 – arguments for a return to the original chronology K. T. Lawrence1, I. Bailey2, and M. E. Raymo3 1Lafayette College, Department of Geology and Environmental Geosciences, 102 Van Wickle Hall, Easton, PA 18042, USA 2National Oceanography Centre Southampton, University of Southampton, European Way, Southampton, SO14 3ZH, UK 3Lamont-Doherty Earth Observatory, P.O. Box 1000, 61 Route 9W, Palisades, NY 10964, USA Correspondence to: K. T. Lawrence ([email protected]) Received: 23 March 2013 – Published in Clim. Past Discuss.: 29 April 2013 Revised: 20 July 2013 – Accepted: 1 October 2013 – Published: 25 October 2013 Abstract. Recently, the veracity of the published chronol- 1 Introduction ogy for the Pliocene section of North Atlantic Ocean Drilling Program Site 982 was called into question. Here, we ex- amine the robustness of the original age model as well as Ocean Drilling Program (ODP) Site 982 was drilled on the ◦ the proposed age model revision. The proposed revision is Rockall Plateau in the subpolar North Atlantic Ocean (58 N, ◦ predicated on an apparent misidentification of the depth to 16 W, at 1134 m water depth) during Leg 162 (Fig. 1). Sed- the Gauss–Matuyama (G/M) polarity chronozone reversal iments recovered from Site 982 have been used to generate boundary (2.581 Ma) based on preliminary shipboard pale- numerous paleoclimate data sets that have made important omagnetic data, and offers a new chronology that includes contributions to our understanding of past oceanic and cli- a hiatus between ∼ 3.2 and 3 Ma. However, an even more matic changes centered on the North Atlantic region (e.g., accurate shore-based, u-channel-derived polarity chronozone Bartoli et al., 2011; Bolton et al., 2011; Lawrence et al., 2009, stratigraphy for the past ∼ 2.7 Ma supports the shipboard 2010; Naafs et al., 2012; Pagani et al., 2009; Sosdian and composite stratigraphy and demonstrates that the original es- Rosenthal, 2009; Venz and Hodell, 2002; Venz et al., 1999). timate of the depth of the G/M reversal in the Site 982 record Recently, Khelifi et al. (2012) have questioned the veracity is correct. Thus, the main justification forwarded to support of the age model for Site 982 that forms the cornerstone of the revised chronology no longer exists. We demonstrate that these works. The original age model for Site 982 was pub- the proposed revision results in a pronounced anomaly in lished in the initial results volume for ODP Leg 162 (Ship- sedimentation rates proximal to the proposed hiatus, erro- board Scientific Party, 1996). This age model was refined by neous assignment of marine-isotope stages in the Site 982 Venz and Hodell (2002) and Venz et al. (1999) using oxy- Pliocene benthic stable oxygen isotope stratigraphy, and a gen isotope stratigraphy and later minimally adjusted, first markedly worse correlation of proxy records between this by Channell and Guyodo (2004) based on new paleomag- site and other regional paleoclimate data. We conclude that netic data, and later when it was incorporated as one of the 18 the original chronology for Site 982 is a far more accurate age records in the global benthic LR04 δ O stack (Lisiecki and model than that which arises from the published revision. We Raymo, 2005) (Fig. 2a). We subsequently collectively refer strongly recommend the use of the original chronology for all to these chronologies, which are negligibly different for the future work at Site 982. Plio-Pleistocene, as the “original age model”. Here, we ex- plore the validity of both the original age model and the revi- sions to it proposed by Khelifi et al. (2012) in their Technical Comment to Climate of the Past (Fig. 2a). To this end, we consider the original shipboard Site 982 physical properties data used to develop the 982 composite splice, the Site 982 Published by Copernicus Publications on behalf of the European Geosciences Union. 2392 K. T. Lawrence et al.: Re-evaluation of the age model for North Atlantic Ocean Site 982 120°W 90°W 60°W 30°W 0° 30°E 60°E Age (ka) 2400 2600 2800 3000 3200 3400 3600 3800 A MG5 MG7 2.5 G3 G7 G17 K1 KM3 M1 G11 G15 G19 MG1 99 101 (‰ PDB) 97 G1 O 3 18 KM6 MG6 3.5 G22 KM2 MG8 102 MG4 2 G16 G20 G2 G6 G10 G14 M2 98 LR04 Original Age Model 4 Site 982 104 2.5 96 G17 K1 MG7 MG11 100 G3 G7 KM3 M1 MG5 4.5 G11 G19 MG1 G15 18 99 101 G1 3 97 O (‰PDB) (! G5 60°N 984 K2 60°N KM6 MG4 3.5 (! 982 G14 G16 G20 KM2 (! 2 102 1314 G6 G10 M2 (! 104 G2 4 (! 2.5 96 98 100 G19 610 G3 G1 G7 G13 M1 611 (! 97 99 101 G11 G17 MG1 4.5 (‰PDB) 3 G5 609 O missing 18 KM4 to K1 3.5 G14 MG4 102 G16 (! G6 G10 G20 4 98 G2 M2 104 1313 Site 982 96 100 G/M, Hole-Aship (Original Hole A = Purple, Hole B = Tan) Khelifi et al. (2012) Age Model 4.5 (Khelifi Hole A=Green, Hole B=Pink, Hole C=Blue) G/M, Hole-BU G/M, Hole-Bship 2400 2600 2800 3000 3200 3400 3600 3800 30°N 30°N 30 sedimentation rate (cm/kyr) original age model B 25 20 15 60°W 30°W 0° 10 Fig. 1. Site map showing the location of ODP Site 982, as well as 5 other relevant North Atlantic sites. 30 0 Figure 1 Khelifi age model C 25 (Hole A=Green, Hole B=Pink, Hole C=Blue) 20 composite splice itself, subsequently generated shore-based 15 u-channel paleomagnetic data (Channell and Guyodo, 2004), 10 δ18 5 and all available Site 982 benthic oxygen isotope ( O) data. sedimentation rate (cm/kyr) 0 We also examine how the paleoclimate data generated from 2400 2600 2800 3000 3200 3400 3600 3800 these sites compare to similar records developed from sed- Age (ka) iments at other localities in the North Atlantic Ocean. In Fig. 2. ODP Site 982 isotopes and sedimentation rates:Figure(A) 2 oxy- this note, we focus on the disputed interval that falls be- gen isotopes from ODP 982 (Lisiecki and Raymo, 2005; Venz and tween 50 and 85 m composite depth (mcd). As detailed be- Hodell, 2002) plotted on the original age model (black) and both low, this evaluation strongly supports the veracity of the orig- the original and Khelifi et al. (2012) isotopes on rmcds of Khelifi et inal Pliocene Site 982 age model. al. (2012) (colors) with associated correlations of both age mod- els with the LR04 oxygen isotope benthic stack (gray) (Lisiecki and Raymo, 2005); (B) sedimentation rates at Site 982 estimated 2 Splices and age models from the original age model (black); (C) sedimentation rates for holes A, B and C estimated from the age model proposed by Khelifi 2.1 Original splice and age model et al. (2012) (colors). Small black arrows and associated labels – lower left-hand corner of (A) – indicate the position of the Gauss– A composite section for Site 982 was generated down to Matuyama chronozone reversal boundary from both shipboard and u-channel measurements. 597 mcd shipboard by splicing together spectral reflectance and GRAPE data that for the Pliocene portion of this record come from holes A and B (Fig. 3). Various age models have been assigned to this stratigraphy by correlation of its ben- 2.2 Khelifi splice and age model thic δ18O for the past ∼ 2.7 Ma to ODP Sites 677 (Channell 18 K0 and Guyodo, 2004; Venz et al., 1999), 846 (Channell and Guided by new benthic foraminiferal δ O data and U37 Guyodo, 2004) and 849 (Flower et al., 2000), for the past SSTs generated on holes A–C, Khelifi et al. (2012) re- 3.2 Ma to Deep Sea Drilling Project (DSDP) Site 607 (Venz vised the shipboard-derived composite depths between 59.1 and Hodell, 2002), and most recently for the past 5.3 Ma to and 84.44 mcd to 59.17 and 85 mcd (hereafter referred to 18 the LR04 stack, following generation of additional δ O data as suggested revised mcd, rmcds), based on 194 new tie between 68.09 and 160.62 mcd (Lisiecki and Raymo, 2005). points (contrast with the four tie points used in the ship- The age model assigned to alkenone-derived sea surface tem- board stratigraphy). Khelifi et al. (2012) justify the genera- perature (SST) data from Site 982 presented in Lawrence et tion of the rmcds on the basis that, between 55 and 72 mcd, al. (2009) is the same one assigned to Site 982 during its in- the shipboard-derived physical properties data are not char- corporation into the LR04 stack (Lisiecki and Raymo, 2005). acterized by oscillations from which reliable correlations can Clim. Past, 9, 2391–2397, 2013 www.clim-past.net/9/2391/2013/ K. T. Lawrence et al.: Re-evaluation of the age model for North Atlantic Ocean Site 982 2393 mcd Original mcd 50 55 60 65 70 75 80 85 982 B % Reflectance (650-700 nm) 50 55 60 65 70 75 80 85 90 100 2 2 140 A splice 6H-6, 97cm 7H-1, 93cm 8H-5, 2cm 9H-1, 101cm 80 120 7H-5, 145cm 8H-1, 21cm 60 2.5 2.5 100 40 80 20 3 3 18 60 O (‰PDB) 0 40 O (‰PDB) 7H-2, 105cm 8H-1, 137cm -20 18 8H-5, 97cm 3.5 3.5 20 7H-4, 137cm -40 6H-4, 25cm 6H-5, 97cm 982 A % Reflectance (650-700 nm) 0 4 4 B splice 8H-2, 38cm 2 1.8 982 B GRAPE Density (g cm ) 4.5 4.5 -3 8H-1, 68cm 8H-6, 144cm 1.9 8H-3, 8H-4, 1.7 50 55 60 65 70 75 80 85 90 10cm 113cm New rmcds (Khelifi) 1.8 1.6 Figure 4 1.7 1.5 Fig.
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