Andrew L.A. Johnson

Pliocene climate and seasonality in North Atlantic shelf seas

This paper reviews North Atlantic shelf seas palaeoclimate during the interval 4–3 Ma, prior to and incorporating the ‘Mid-Pliocene warm period’ (ca 3.29–2.97 Ma). Fossil assemblages and stable isotope data demonstrate northwards extension of subtropical faunas along the coast of the Carolinas–Virginia (Yorktown and Duplin Formations) relative to the present day, suggesting a more vigorous Florida Current, with reduced seasonality and warm water extending north of Cape Hatteras (reconstructed annual range for Virginia 12–30°C). This interpretation supports conceptual models of increased meridional heat transport for the Pliocene. Sea temperatures for Florida (Lower Pinecrest Beds) were similar to or slightly cooler than (summers 25–27°C) today, and were probably influenced by seasonal upwelling of cold deep water. Reduced seasonality is also apparent in the Coralline Crag Formation of the southern North Sea, with ostracods suggesting winter sea temperatures of 10°C (modern 4°C). However, estimates from Pliocene bivalves (3.6–16.6°C) are similar to or cooler than the present day. This ‘mixed’ signal is problematic given warmer seas in the Carolinas–Virginia, and climate model and oceanographic data that show warmer seas in the ‘Mid-Pliocene’ eastern North Atlantic. This may be because the Coralline Crag Formation was deposited prior to peak Mid-Pliocene warmth.

The PRISM (Pliocene palaeoclimate) reconstruction: time for a paradigm shift.

Global palaeoclimate reconstructions have been invaluable to our understanding of the causes and effects of climate change, but single-temperature representations of the oceanic mixed layer for data–model comparisons are outdated, and the time for a paradigm shift in marine palaeoclimate reconstruction is overdue. The new paradigm in marine palaeoclimate reconstruction stems the loss of valuable climate information and instead presents a holistic and nuanced interpretation of multi-dimensional oceanographic processes and responses. A wealth of environmental information is hidden within the US Geological Surveys Pliocene Research, Interpretation and Synoptic Mapping (PRISM) marine palaeoclimate reconstruction, and we introduce here a plan to incorporate all valuable climate data into the next generation of PRISM products. Beyond the global approach and focus, we plan to incorporate regional climate dynamics with emphasis on processes, integrating multiple environmental proxies wherever available in order to better characterize the mixed layer, and developing a finer time slice within the Mid-Piacenzian Age of the Pliocene, complemented by underused proxies that offer snapshots into environmental conditions. The result will be a proxy-rich, temporally nested, process-oriented approach in a digital format—a relational database with geographic information system capabilities comprising a three-dimensional grid representing the surface layer, with a plethora of data in each cell.

Evolution of European lower Jurassic Gryphaea (Gryphaea) and contemporaneous bivalves

Standardised data, representing ‘adulthood’, reveal that in characters of gross shape, shell thickness and size G. (Gryphaea) exhibits marked changes through the European Lower Jurassic. Evolution, evidently anagenetic, was not certainly unidirectional and even‐paced (truly gradualistic) in any character — for some characters brief periods of stasis may have occurred. However, few, if any, intervals were marked by stasis in all characters, and no horizons by especially conspicuous change. Punctuated equilibrium is thus not displayed. Parallel patterns of gross shape, thickness and size change can be readily integrated in an ‘adaptational’ model of evolution upon appreciation that each variable is likely to affect shell stability, a factor influencing survival in reclining bivalves. Thus periods of evolution towards a more stable gross shape (broader, flatter) are considered to be a selectively‐driven response to coeval size and thickness changes (reductions) tending to diminish stability in a shell poorly...

Evidence and cause of small size in Bathonian (Middle Jurassic) marine bivalves of north‐western Europe

Data from the monographic literature indicate a general size reduction amongst Bathonian marine bivalves in southern England: mean size is reduced in all subclasses and major mode-of-life categories relative to the immediately preceding Bajocian stage (27 per cent. reduction) and the later Oxfordian stage (41 per cent. reduction). Smaller size cannot be explained in terms of sedimentary facies, nor as a ‘pure’ evolutionary phenomenon (involving no environmental change); it therefore probably reflects some quality of the ambient water. A comparable reduction in average size (19 per cent. relative to Bajocian, 46 per cent. relative to Oxfordian) is evident amongst oyster and scallop lineages studied throughout Europe and a sedimentary-facies (and pure evolutionary) control can be ruled out at least for an area extending from southern England to the east side of the Paris Basin. A lowering of salinity to the mid-twenties per mil is the likeliest cause of size reduction in this area. Isolation of the region from the oceans, combined with high fresh water runoff, was probably the determining factor in such widespread lowering of salinity. High runoff may have been caused by increased rainfall. Tests are proposed for the ubiquity of the pattern identified, and the suggested proximate and ultimate causes. It is noted that recognition of widespread reduced salinity has implications for the salinity tolerances of certain ‘stenohaline-marine’ taxa and the salinity ranges ascribed to certain nearshore faunal associations. In addition, recognition that environmentally determined size reduction may occur over a broad area has implications for the general issue of interpreting phyletic size change.

Punctuated equilibria versus phyletic gradualism in European Jurassic Gryphaea evolution

Analysis of new and existing data indicates that the pattern of European Jurassic Gryphaea evolution is neither uniquely concordant with the punctuated equilibria model nor the phyletic gradualism model. Phylogeny involves significant intervals of both stasis and more or less continuous evolution (not necessarily involving the same characters and thus not necessarily representing classical gradualism). However, change may be more characteristic of Gryphaea than stable equilibrium. New data favour the idea that Lower Jurassic Gryphaea evolved from Triassic congeners, rather than early Jurassic representatives of Liostrea .

ISOTOPIC TEMPERATURES FROM THE EARLY AND MID-PLIOCENE OF THE US MIDDLE ATLANTIC COASTAL PLAIN, AND THEIR IMPLICATIONS FOR THE CAUSE OF REGIONAL MARINE CLIMATE CHANGE

Abstract: Mean seasonal extreme temperatures on the seafloor calculated from the shell δ18O of the scallop Placopecten clintonius from the basal part of the early Pliocene Sunken Meadow Member (Yorktown Formation) in Virginia are very similar to those from the same horizon at the latitude of Cape Hatteras in North Carolina (∼ 210 km to the south). The lowest and highest temperatures calculated from each shell (using δ18Oseawater = +0.7‰) give mean values for winter and summer of 8.4 ± 1.1 °C (± 1σ) and 18.2 ± 0.6 °C in Virginia, and 8.6 ± 0.4 °C and 16.5 ± 1.1 °C in North Carolina (respective median temperatures: 13.3 °C and 12.6 °C). Patterns of ontogenetic variation in δ18O, δ13C and microgrowth increment size indicate summer water-column stratification in both areas, with summer surface temperatures perhaps 6 °C higher than on the seafloor. The low winter paleotemperatures in both areas are most simply explained by the greater southward penetration of cool northern waters in the absence of a feature equivalent to Cape Hatteras. The same current configuration but a warmer general climate can account for the high benthic seasonal range (over 15.0 °C in some cases) but warmer median temperatures (15.7–21.3 °C) derived from existing δ18O data from scallops of the higher Yorktown Formation (using δ18Oseawater = +0.7‰ for the upper Sunken Meadow Member and δ18Oseawater = +1.1‰ for the mid-Pliocene Rushmere, Morgarts Beach, and Moore House members). Existing δ18O data from the infaunal bivalve Mercenaria of the Rushmere Member yields a similarly high median temperature (21.6 °C) but a low seasonal range (9.2 °C), pointing to the periodic influence of warm currents, possibly at times when the Gulf Stream was exceptionally vigorous.