Cretaceous polar conifer forests : composition, leaf life-span and climate significance

Fossil wood is abundant in sediments that were deposited at high latitudes during the Cretaceous. The wood provides evidence for forests growing in high latitudes in the past, at times when climate was warmer during greenhouse periods. However, computer climate models that try to simulate past climates cannot reproduce the polar warmth necessary for the existence of forests but more commonly generate ice-covered polar regions. This may be due to the lack of a realistic vegetation component in the models. The aim of this project is to determine the leaf habit and leaf life-spans (LLSs) of conifers growing at high latitudes during the mid-Cretaceous. The results will be used to test the outputs of the University of Sheffield Conifer Model (USCM), a model that will produce a new reconstruction of high latitude vegetation for coupling with computer climate models to determine the effects of high latitude vegetation on climate systems in the past. Fossil conifer wood is studied from mid-Cretaceous sediments from the Canadian Arctic, Svalbard, Australia and Antarctica, all high latitude sites during the mid-Cretaceous. Identification of the woods indicates that Northern Hemisphere forests were composed of Piceoxylon, Laricioxylon, Cedroxylon, Taxodioxylon, Pinuxylon, Palaepiceoxylon, Taxoxylon, Juniperoxylon, Protocedroxylon, Araucariopitys, Xenoxylon and Cupressinoxylon. The Canadian Arctic was dominated by Pinuxylon and Svalbard by Taxodioxylon. In the Southern Hemisphere the conifers include Podocarpoxylon, Taxodioxylon, Araucariopitys, Cupressinoxylon, Agathoxylon, Protophyllocladoxylon, Sciadopityoxylon and Circoporoxylon. Antarctica was dominated by Taxodioxylon and in Australia the woods were dominated by Podocarpoxylon. Palaeoclimate analysis using nearest living relative and growth ring analyses indicate that the mid-Cretaceous climate in the Canadian Arctic was cool temperate and Svalbard was cool to warm temperate. In the Southern Hemisphere Australia appears to have been cool temperate whilst in Antarctica it is only possible to define the climate as temperate. A new technique was developed to determine the leaf habit (evergreen or deciduousness) and LLSs of modern conifers using wood anatomy. This method quantitatively characterises cell patterns within growth rings as the Ring Markedness Index (RMI, a measure of the markedness of the ring boundaries). The RMI has been shown to have a strong logarithmic relationship to leaf life-span and can therefore be used to determine the leaf life-span of fossil woods. Results of RMI analysis on fossil conifer woods provide an indication of the LLSs of mid-Cretaceous conifers for the first time. Both northern and southern hemisphere forests were dominated by evergreen conifers, with only a small component of deciduous conifer types. The dominant LLS in both Northern Hemisphere sites (the Canadian Arctic and Svalbard) was 33 months. In the Southern Hemisphere the dominant LLS in Australia was 63 months, on James Ross Island, Antarctica 48 months and on Alexander Island, Antarctica it was 151 months. Comparison with leaf habit and LLS predictions from USCM indicate a reasonable match, verifying model output. The Conifer Model will now be coupled with regional and global climate models to examine the magnitude of the effect of conifer forests in the polar regions during the mid-Cretaceous greenhouse climat

The formation and inversion of the Western Greater Caucasus basin and the uplift of the western greater Caucacus: implications for the wider black sea region

The western Greater Caucasus formed by the tectonic inversion of the western strand of the Greater Caucasus Basin, a Mesozoic rift that opened at the southern margin of Laurasia. Subsidence analysis indicates that the main phase of rifting occurred during the Aalenian to Bajocian synchronous with that in the eastern Alborz and, possibly, the South Caspian Basin. Secondary episodes of subsidence during the late Tithonian to Berriasian and Hauterivian to early Aptian are tentatively linked to initial rifting within the western, and possibly eastern, Black Sea and during the late Campanian to Danian to the opening of the eastern Black Sea. Initial uplift, subaerial exposure, and sediment derivation from the western Greater Caucasus occurred at the Eocene-Oligocene transition. Oligocene and younger sediments on the southern margin of the former basin were derived from the inverting basin and uplifted parts of its northern margin, indicating that the western Greater Caucasus Basin had closed by this time. A predominance of pollen representing a montane forest environment (dominated by Pinacean pollen) within these sediments suggests that the uplifting Caucasian hinterland had a paleoaltitude of around 2 km from early Oligocene time. The closure of the western Greater Caucasus Basin and significant uplift of the range at approximately 34 Ma is earlier than stated in many studies and needs to be incorporated into geodynamic models for the Arabia-Eurasia region

Hadley circulation and precipitation changes control black shale deposition in the Late Jurassic Boreal Seaway

New climate simulations using the HadCM3L model with a paleogeography of the Late Jurassic [155.5 Ma], and proxy-data corroborate that warm and wet tropical-like conditions reached as far north as the UK sector of the Jurassic Boreal Seaway [~35oN]. This is associated with a northern hemisphere Jurassic Hadley cell and an intensified subtropical jet which both extend significantly polewards than in the modern (July-September). Deposition of the Kimmeridge Clay Formation [KCF] occurred in the shallow, storm-dominated, epeiric Boreal Seaway. High resolution paleo-environmental proxy data from the Kimmeridge Clay Formation [KCF; ~155–150 Ma], UK are used to test for the role of tropical atmospheric circulation on meter-scale heterogeneities in black shale deposition. Proxy and model data show that the most organic-rich section [eudoxus to mid-hudlestoni zones] is characterised by a positive δ13Corg excursion and up to 37 wt% total organic carbon [%TOC]. Orbital-modulation of organic carbon burial primarily in the long eccentricity power band combined with a clear positive correlation between %TOC carbonate-free and the kaolinite/illite ratio supports peak organic carbon burial under the influence of very humid climate conditions, similar to the modern tropics. This re-interpretation of large-scale climate relationships, supported by independent modelling and geological data, has profound implications for atmospheric circulation patterns and processes affecting marine productivity and organic carbon burial further north along the Boreal Seaway, including the Arctic

Structure and diversity of the Mesozoic wood genus <i>Xenoxylon</i> in Far East Asia: implications for terrestrial palaeoclimates

Although the faunal elements of Far East Asian Mesozoic terrestrial biota have attracted much attention in recent years, their palaeoecology remains poorly known. In particular, features of the palaeoclimate are highly controversial. To address this point we used the Mesozoic fossil wood Xenoxylon, a genus recognized as an indicator of wet temperate biotopes and which is common in the area during the Carnian–Maastrichtian interval. We re-appraised bibliographic data and gathered new data for Xenoxylon in the Mesozoic of Far East Asia. This demonstrated that previous taxonomic approaches to the genus have been so far idiosyncratic. We examined the anatomical diversity of morphogenus Xenoxylon in Far East Asia and compared it to that of samples from Europe. This indicates that in an area centred on north-eastern China, Xenoxylon reached a level of anatomical diversity unmatched elsewhere in the world. We hypothesize that this diversity witnesses the persistence of palaeoecological conditions particularly suitable for Xenoxylon and that a wet temperate climate prevailed over most of the area throughout the Carnian–Maastrichtian interval. It is in this setting that the famous Jehol Biota probably evolved

Which name(s) should be used for Araucaria-likefossil wood? – Results of a poll

Araucarioxylon Kraus is a widely known fossil-genus generally applied to woods similar to that of the extant Araucariaceae. However, since 1905, several researchers have pointed out that this name is an illegitimate junior nomenclatural synonym. At least four generic names are in current use for fossil wood of this type: Agathoxylon Hartig, Araucarioxylon, Dadoxylon Endl. and Dammaroxylon J.Schultze-Motel. This problem of inconsistent nomenclatural application is compounded by the fact that woods of this type represent a wide range of plants including basal pteridosperms, cordaitaleans, glossopterids, primitive conifers, and araucarian conifers, with a fossil record that extends from the Devonian to Holocene. Conservation of Araucarioxylon has been repeatedly suggested but never officially proposed. Since general use is a strong argument for conservation, a poll was conducted amongst fossil wood anatomists in order to canvass current and preferred usage. It was found that the community is divided, with about one-fifth recommending retention of the well-known Araucarioxylon, whereas the majority of others advocated use of the legitimate Agathoxylon. The arguments of the various colleagues who answered the poll are synthesized and discussed. There is clearly little support for conservation of Araucarioxylon. A secondary aspect of the poll tackled the issue as to whether Araucaria-like fossil woods should be either gathered into a unique fossil-genus, or whether two fossil-genera should be recognized, based on the respective presence or absence of axial parenchyma. A majority of colleagues favoured having one fossil-genus only. Agathoxylon can be used legitimately and appears to be the most appropriate name for such woods. However, its original diagnosis must be expanded if those woods lacking axial parenchyma are to be included