Calcium isotopic composition of high-latitude proxy carrier Neogloboquadrina pachyderma (sin.)

International audienceThe accurate reconstruction of sea surface temperature (SST) history in climate-sensitive regions (e.g. tropical and polar oceans) became a challenging task in palaeoceanographic research. However, biogenic shell carbonate SST proxies successfully developed for tropical regions often fail in cool water environments. Their major regional shortcomings and the cryptic diversity now found within the major high latitude proxy carrier Neogloboquadrina pachyderma (sin.) highlight an urgent need to develop complementary SST proxies for these cool water regions. Here we incorporate the genetic component into a calibration study of a new SST proxy for the high latitudes. We found that the calcium isotopic composition (?44/40Ca) of calcite from genotyped net catches and core-top samples of the planktonic foraminifera Neogloboquadrina pachyderma (sin) is strongly related to temperature and unaffected by genetic variations. The temperature sensitivity has been found to be 0.17 (±0.04)? per 1°C highlighting its potential for downcore applications in open marine cool-water environments. Our results further indicate that however in extreme polar environments, below a critical threshold temperature of 2.0 (±0.5)°C and salinity of 33.0 (±0.5)? a prominent shift in biomineralization affect the Ca isotope composition of N. pachyderma (sin.) becoming insensitive to temperature. These findings highlight the need of systematic calibration studies to unravel the influencing factors on Ca isotope fractionation and to validate the proxies' applicability

Genotypic variability in subarctic Atlantic planktic foraminifera

Specimens of the planktic foraminiferal morphospecies, Globigerina bulloides, Turborotalita quinqueloba, Neogloboquadrina pachyderma (dextral) and Globigerinita uvula, were collected along a subarctic Atlantic transect. Partial sequences of the small subunit (SSU) ribosomal (r) RNA gene were obtained and a distance-based foraminiferal phylogeny constructed. The low latitude morphospecies, Globigerina falconensis, was included to improve within cluster resolution. G. bulloides, G. falconensis and T. quinqueloba cluster together as a distinct group within the molecular phylogeny. The diversification of these three morphospecies from their common ancestor is clearly later than the main planktic spinose radiation, consistent with current interpretations of the fossil record. G. bulloides and G. falconensis are highly divergent from one another, supporting palaeontological and biological evidence that they are separate species. N. pachyderma (dextral) clusters with Neogloboquadrina dutertrei within the benthic and non-spinose planktic region of the tree. G. uvula also clusters within the benthic and non-spinose planktic region of the tree, adjacent to Globigerinita glutinata, a member of the same genus, though resolution is too low to provide evidence of a sister-taxon relationship. The Globigerina bulloides and Turborotalita quinqueloba morphospecies comprise a complex of distinct SSU rDNA genetic types. These fall into two groups, representing high and low latitude genotypes. Along the subarctic transect, G. bulloides and T. quinqueloba were each represented by two distinct genotypes. Neogloboquadrina pachyderma (dextral) and Globigerinita uvula were each represented by a single genotype. Genotypes of a morphospecies exhibit distinctive and different distribution patterns. In the case of Globigerina bulloides, the genotype distribution is suggestive of differing adaptation. However, the Turborotalita quinqueloba genotype distribution was complicated by their co-existence in the same water column throughout the eastern sector. Further investigation will be required to determine whether they occupy a different niche within the water column. Although only T. quinqueloba Type IIa was found in the western region, sampling density was low and inconclusive. The Neogloboquadrina pachyderma (dextral) genotype was found across the entire transect. Further investigation of genotype distribution and genotype/habitat relationships could provide new high-resolution proxies for past oceanographic/climate reconstructions. © 2001 Elsevier Science B.V. All rights reserved