Reply on CC1

Andrew Johnson

doi:10.5194/cp-2021-142-ac3

Abstract

Abstract. Oxygen isotope (Î´18O) sclerochronology of benthic marine molluscs provides a means of reconstructing the seasonal range in seafloor temperature, subject to use of an appropriate equation relating shell Î´18O to temperature and water Î´18O, a reasonably accurate estimation of water Î´18O, and due consideration of growth-rate effects. Taking these factors into account, Î´18O data from late Pliocene bivalves of the southern North Sea basin (Belgium and the Netherlands) indicate a seasonal seafloor range a little smaller than now in the area. Microgrowth-increment data from Aequipecten opercularis, together with the species composition of the bivalve assemblage and aspects of preservation, suggest a setting below the summer thermocline for all but the latest material investigated. This implies a higher summer temperature at the surface than on the seafloor and consequently a greater seasonal range. A reasonable (3ââC) estimate of the difference between maximum seafloor and surface temperature under circumstances of summer stratification points to seasonal surface ranges in excess of the present value (12.4ââC nearby). Using a model-derived estimate of water Î´18O (0.0ââ°), summer surface temperature was initially in the cool temperate range (<20ââC) and then (during the Mid-Piacenzian Warm Period; MPWP) increased into the warm temperate range (>20ââC) before reverting to cool temperate values (in conjunction with shallowing and a loss of summer stratification). This pattern is in agreement with biotic-assemblage evidence. Winter temperature was firmly in the cool temperate range (<10ââC) throughout, contrary to previous interpretations. Averaging of summer and winter surface temperatures for the MPWP provides a figure for annual sea surface temperature that is 2â3ââC higher than the present value (10.9ââC nearby) and in close agreement with a figure obtained by averaging alkenone and TEX86 temperatures for the MPWP from the Netherlands. These proxies, however, respectively, underestimate summer temperature and overestimate winter temperature, giving an incomplete picture of seasonality. A higher annual temperature than now is consistent with the notion of global warmth in the MPWP, but a low winter temperature in the southern North Sea basin suggests regional reduction in oceanic heat supply, contrasting with other interpretations of North Atlantic oceanography during the interval. Carbonate clumped isotope (Î47) and biomineral unit thermometry offer means of checking the Î´18O-based temperatures.

Highlights

1 Introduction The foraminiferal δ18O record from the deep sea indicates that the global volume of land ice 45 was generally lower than during the Pliocene Epoch (Lisiecki and Raymo, 2005), and that global mean surface temperature (GMST) was generally higher
The Merksem-equivalent shells yield low winter temperatures but summer temperatures well below those indicated by Oorderen and equivalent shells, and below those typical of the southern North Sea at present, though it has to be acknowledged that the two shells investigated may not be representative of their time
The summer seafloor temperatures recorded from a late Pliocene A. islandica specimen (18.2 and 19.9 °C using water δ18O values of 0.0 and +0.4 ‰, respectively) corroborate evidence supplied by an early Pleistocene example from Italy (20.3 °C from specimen 920 ACG254-1 using a water δ18O value of +0.5 ‰; Crippa et al, 2016, table 1) of a thermal range greater in the past than at present

Summary

15 Abstract

Oxygen isotope (δ18O) sclerochronology of benthic marine molluscs provides a means of reconstructing the seasonal range in seafloor temperature, subject to use of an appropriate equation relating shell δ18O to temperature and water δ18O, reasonably accurate estimation of water δ18O, and due consideration of growth-rate effects Taking these factors into account, δ18O data from late Pliocene bivalves of the southern North Sea Basin Microgrowth-increment data from Aequipecten opercularis, together with the speciescomposition of the bivalve assemblage and aspects of preservation, suggest a setting below the summer thermocline for all but the latest material investigated This implies a higher summer temperature at the surface than on the seafloor and a greater seasonal range. Carbonate clumped isotope (Δ47) and biomineral unit thermometry offer means of checking the δ18O-based temperatures

Introduction

Laboratory procedures

Calculation of temperatures 475 In previous work on late Pliocene bivalves from

Seasonal seafloor ranges With the exception of the

Meaning of δ13C data 840

Conclusions