PALAEOXYLOTOMICAL STUDIES IN THE CENOZOIC PETRIFIED FORESTS OF GREECE. PART TWO – CONIFERS

This paper reports the palaeoxylotomical study of petrified conifer remains from Velitzelos collection, orig-inating from some fossiliferous sites of Greece, especially from the Aegean area (Evros, Limnos, Lesbos), aged of late Oligocene to early Miocene. Ten species were identified: Cupressinoxylon akdikii, Juniperoxylon acarcae, Tetraclinoxylon velitzelosii, Taxodioxylon gypsaceum, Taxodioxylon taxodii, Glyptostroboxylon rudolphii, Glyptostroboxylon tenerum, Pinuxylon pineoides, Pinuxylon halepensoides and Pinuxylon sp. aff. Pinus canariensis. These new identifications add new elements to the forest assemblages of the Oligocene - Miocene Greek flora, useful for understanding the evolution of the Cenozoic palaeovegetation and palaeoclimate in the Aegean area.

Cenozoic Antarctic climate evolution based on molecular and isotopic biomarker reconstructions from geological archives in the Ross Sea region

<p>During the Cenozoic Era (the last 65 Ma), Antarctica’s climate has evolved from ice free conditions of the ‘Greenhouse world’, which at its peak (~ 55 Ma) supported near-tropical forests, to the ‘Icehouse’ climate of today with permanent ice sheets, and a very sparse macroflora. This long-term cooling trend is punctuated by a number of major, abrupt, and in some cases, irreversible climate transitions. Reconstructing past changes in vegetation, sea surface temperature, hydroclimate and the carbon cycle require robust geological proxies that in turn can provide insights into climatic thresholds and feedbacks that drove major transitions in the evolution of Antarctica’s ice sheets. Biomarkers allow climate and environmental proxy reconstructions for this region, where other more traditional paleoclimate methods are less suitable. This study has two aims. Firstly to assess the suitability and applicability of biomarkers in Antarctic sediments across a range of depositional settings and ages, and secondly to apply biomarker-based climate proxies to reconstruct environmental and climate conditions during key periods in the development of the Antarctic Ice Sheets. The distribution and abundances of n-alkanes are assessed in Oligocene and Miocene sediments from a terrestrial outcrop locality in the Transantarctic Mountains, and two glaciomarine sediment cores and an ice-distal deep marine core from the western Ross Sea. Comparisons are made with n-alkane distributions in Eocene glacial erratics and sedimentary rocks of the Mesozoic Beacon Supergroup, both likely sources of reworked material. A shift in dominant chain length from n-C₂₉ to n-C₂₇ occurs between the Late Eocene and Early Oligocene, considered a response to a significant climate cooling. Samples from glaciofluvial environments onshore, and subglacial and ice-proximal environments offshore display a reworked n-alkane distribution, characterised by low carbon preference index (CPI), high average chain length (ACL) and high n-C₂₉/n-C₂₇ values. Whereas, samples from lower-energy, more benign lacustrine and ice-distal marine environments predominantly contained contemporary material. Palynomorphs and biomarker proxies based on n-alkanes and glycerol dialkyl glycerol tetraethers (GDGTs) are applied to a Late Oligocene and Early Miocene glaciomarine succession spanning the large transient excursion of the Mi-1 glaciation (~23 Ma) in DSDP Site 270 drill core from the central Ross Sea. While the Late Oligocene is marked by relatively warm conditions, regional cooling initiated a transition into Mi-1. This was likely driven by a combination of decreasing atmospheric CO₂ and an orbital geometry favouring low seasonality and cool summers, leading to an intensification of proto-Antarctic bottom water production as the Ross Sea deepened and cooled. Mi-1 manifests as a regionally cool period, with minimum subsurface temperatures of ~4°C and onshore mean summer temperatures of ~8°C. A negative n-alkane δ¹³C excursion of up to 4.8‰ is interpreted as a vegetation response to cold, restricted growing seasons, with plants driven to lower altitudes and more stunted growth forms. However, ocean temperatures remained too warm for marine-based ice sheets to advance onto the outer continental shelf and over-ride the drill site. The large increase in ice volume associated with this event, implied by global δ¹⁸O records, was probably held on a higher, terrestrial West Antarctica of greater extent than present day. The relative lack of ice rafted debris during Mi-1, suggests the presence of a marginal marine-terminating ice sheet with fringing ice shelves to the south of DSDP site 270, calving icebergs lacking a basal debris layer, similar to those calving from the Ross Ice Shelf today. This extensive ice cover may explain a large decrease in marine n-alkanes at this time restricting marine productivity on the continental shelf. The biomarker data for the Early Miocene in DSDP 270 indicates a relative warming in both terrestrial and marine temperatures following the transient Mi-1 glacial expansion, but an overall baseline cooling of climate between Late Oligocene and the Early Miocene in the Ross Sea embayment. Isoprenoid GDGTs are used to reconstruct a Cenozoic subsurface ocean temperature compilation for the Ross Sea, a key source region of ocean deep water. The ocean temperature TEXL86 calibration and BAYSPAR in standard subsurface mode were considered, through comparison with independent microfossil and sedimentological data, the most appropriate for use in this region. Ocean temperatures cool prior to the Eocene/Oligocene transition and remain cool for the rest of the Cenozoic, with the exception of short periods of relative warmth in the Late Oligocene and Mid-Miocene Climate Optimum, and long-term trends broadly mirror that of the foraminiferal δ¹⁸O record from the deep Pacific. The Δ Ring Index is used to assess non-thermal influences on GDGT distributions, and displays a long term shift from more positive to more negative deviations. This correlates with %GDGT-0, and also relates to a declining trend in the Methane Index, which reflect the contribution of methanogenic and methanotrophic archaea. These changes suggest that these archaea contributed more to the archaeal community in the early to mid Cenozoic, potentially indicating a more anoxic depositional environment in the Ross Sea. The Branched to Isoprenoid Tetraether index (BIT) steadily declines over the Cenozoic, reflecting increasingly hyper-arid conditions onshore, with less active glaciofluvial systems, limited soil development and less ice-free land.</p>

Cenozoic Antarctic climate evolution based on molecular and isotopic biomarker reconstructions from geological archives in the Ross Sea region

<p>During the Cenozoic Era (the last 65 Ma), Antarctica’s climate has evolved from ice free conditions of the ‘Greenhouse world’, which at its peak (~ 55 Ma) supported near-tropical forests, to the ‘Icehouse’ climate of today with permanent ice sheets, and a very sparse macroflora. This long-term cooling trend is punctuated by a number of major, abrupt, and in some cases, irreversible climate transitions. Reconstructing past changes in vegetation, sea surface temperature, hydroclimate and the carbon cycle require robust geological proxies that in turn can provide insights into climatic thresholds and feedbacks that drove major transitions in the evolution of Antarctica’s ice sheets. Biomarkers allow climate and environmental proxy reconstructions for this region, where other more traditional paleoclimate methods are less suitable. This study has two aims. Firstly to assess the suitability and applicability of biomarkers in Antarctic sediments across a range of depositional settings and ages, and secondly to apply biomarker-based climate proxies to reconstruct environmental and climate conditions during key periods in the development of the Antarctic Ice Sheets. The distribution and abundances of n-alkanes are assessed in Oligocene and Miocene sediments from a terrestrial outcrop locality in the Transantarctic Mountains, and two glaciomarine sediment cores and an ice-distal deep marine core from the western Ross Sea. Comparisons are made with n-alkane distributions in Eocene glacial erratics and sedimentary rocks of the Mesozoic Beacon Supergroup, both likely sources of reworked material. A shift in dominant chain length from n-C₂₉ to n-C₂₇ occurs between the Late Eocene and Early Oligocene, considered a response to a significant climate cooling. Samples from glaciofluvial environments onshore, and subglacial and ice-proximal environments offshore display a reworked n-alkane distribution, characterised by low carbon preference index (CPI), high average chain length (ACL) and high n-C₂₉/n-C₂₇ values. Whereas, samples from lower-energy, more benign lacustrine and ice-distal marine environments predominantly contained contemporary material. Palynomorphs and biomarker proxies based on n-alkanes and glycerol dialkyl glycerol tetraethers (GDGTs) are applied to a Late Oligocene and Early Miocene glaciomarine succession spanning the large transient excursion of the Mi-1 glaciation (~23 Ma) in DSDP Site 270 drill core from the central Ross Sea. While the Late Oligocene is marked by relatively warm conditions, regional cooling initiated a transition into Mi-1. This was likely driven by a combination of decreasing atmospheric CO₂ and an orbital geometry favouring low seasonality and cool summers, leading to an intensification of proto-Antarctic bottom water production as the Ross Sea deepened and cooled. Mi-1 manifests as a regionally cool period, with minimum subsurface temperatures of ~4°C and onshore mean summer temperatures of ~8°C. A negative n-alkane δ¹³C excursion of up to 4.8‰ is interpreted as a vegetation response to cold, restricted growing seasons, with plants driven to lower altitudes and more stunted growth forms. However, ocean temperatures remained too warm for marine-based ice sheets to advance onto the outer continental shelf and over-ride the drill site. The large increase in ice volume associated with this event, implied by global δ¹⁸O records, was probably held on a higher, terrestrial West Antarctica of greater extent than present day. The relative lack of ice rafted debris during Mi-1, suggests the presence of a marginal marine-terminating ice sheet with fringing ice shelves to the south of DSDP site 270, calving icebergs lacking a basal debris layer, similar to those calving from the Ross Ice Shelf today. This extensive ice cover may explain a large decrease in marine n-alkanes at this time restricting marine productivity on the continental shelf. The biomarker data for the Early Miocene in DSDP 270 indicates a relative warming in both terrestrial and marine temperatures following the transient Mi-1 glacial expansion, but an overall baseline cooling of climate between Late Oligocene and the Early Miocene in the Ross Sea embayment. Isoprenoid GDGTs are used to reconstruct a Cenozoic subsurface ocean temperature compilation for the Ross Sea, a key source region of ocean deep water. The ocean temperature TEXL86 calibration and BAYSPAR in standard subsurface mode were considered, through comparison with independent microfossil and sedimentological data, the most appropriate for use in this region. Ocean temperatures cool prior to the Eocene/Oligocene transition and remain cool for the rest of the Cenozoic, with the exception of short periods of relative warmth in the Late Oligocene and Mid-Miocene Climate Optimum, and long-term trends broadly mirror that of the foraminiferal δ¹⁸O record from the deep Pacific. The Δ Ring Index is used to assess non-thermal influences on GDGT distributions, and displays a long term shift from more positive to more negative deviations. This correlates with %GDGT-0, and also relates to a declining trend in the Methane Index, which reflect the contribution of methanogenic and methanotrophic archaea. These changes suggest that these archaea contributed more to the archaeal community in the early to mid Cenozoic, potentially indicating a more anoxic depositional environment in the Ross Sea. The Branched to Isoprenoid Tetraether index (BIT) steadily declines over the Cenozoic, reflecting increasingly hyper-arid conditions onshore, with less active glaciofluvial systems, limited soil development and less ice-free land.</p>

Thermo-tectonic studies of Mesozoic basement rocks, North Island, New Zealand

<p>The basement rocks of North Island, New Zealand, comprise metasedimentary terranes that were accreted onto the eastern Gondwana margin during Mesozoic subduction. Since the Oligocene, these terranes have been sitting at the leading edge of the Australian Plate, as the hanging wall of the Hikurangi subduction margin, overriding the subducting Pacific Plate. This thesis examines the thermo-tectonic histories of the basement rocks in North Island, using fission-track and (U-Th-Sm)/He thermochronology. In eastern North Island, thermochronological data from the basement rocks record the exhumation histories since the latest Jurassic, related to two subduction cycles. Zircon fission-track analysis yields detrital or slightly reset ages (264–102 Ma); apatite fission-track ages range from 122 to 7.9 Ma and (U-Th-Sm)/He from 33.3 to 6.0 Ma. In central North Island, modelled thermal histories suggest that the basement rocks were exhumed to shallow levels (<2 km) of the crust in the Early Cretaceous (~150–135 Ma). This was followed by a period of reheating until ~100 Ma, which is interpreted to be the result of burial by sedimentation above the accretionary wedge. From 100 Ma, models indicate thermo-tectonic quiescence until the Late Oligocene. During the late Cenozoic, exhumation of the basement rocks accelerated at ~27 Ma in the western margin of the axial ranges (Kaimanawa Mountains). This acceleration in exhumation rate is interpreted to reflect the initiation of the subduction of the Pacific Plate beneath central North Island. Since the Late Oligocene, basement exhumation in the axial ranges migrated towards the trough. Modelled thermal histories indicate significant eastwards reverse faulting on the margin-parallel Ngamatea Fault between ~27 and 20 Ma and on the Wellington-Mohaka Fault between ~20 and 10 Ma. In contrast to the activity in the axial ranges, in western North Island, the exhumational response of the basement rocks to the Cenozoic subduction was less significant and not revealed from the present thermochronological data. Since the Late Miocene, the exhumation rate in the axial ranges has varied significantly along-strike, lower in the centre and higher to the north and south. During the last 10 Myr, the total magnitude of exhumation has been ~4 km in the Wellington region in the south, >1 km in the Raukumara Range in the north and negligible (less than a few hundred metres) in the central axial ranges in the Hawke’s Bay region. Although the accumulation of underplated material at the basal upper plate may have contributed to the localised rock uplift and exhumation (e.g. in the Raukumara Range), margin-normal shortening of the upper plate in the forearc of the Hikurangi Margin has most likely dominated the unroofing process of the axial ranges. In northwestern North Island, the Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto in situ Mesozoic and early Cenozoic rocks. Detrital zircon and apatite fission-track ages reveal that the basal Northland Allochthon sequences and the underlying Miocene autochthonous sedimentary rocks were predominantly derived from the local Jurassic terrane (Waipapa Supergrop) and perhaps the Late Cretaceous volcanics. In addition, the Early Miocene autochthon contains significant sedimentary influx from the Late Oligocene volcanics related to the subduction initiation in northern New Zealand. Zircon and apatite fission-track data from the in situ Mesozoic basement were inverted using thermo-kinematic models coupled with an inversion algorithm. The results suggest that during the Late Oligocene, ~4–6 km thick nappes were emplaced onto the in situ rocks in the northernmost Northland region. Prior to basement unroofing in the Early Miocene, the nappes thinned towards the south. Following allochthon emplacement, eastern Northland was uplifted and unroofed rapidly over a period of ~1–6 Myr, leading to ~0.4–1.5 km erosion of the allochthon. Since the mid-Miocene, due to the decline in tectonic activity in this region, the Northland Allochthon and the underlying rocks have been eroded slowly. This thesis has documented variable exhumation and burial processes that occurred in the upper plates of both the Mesozoic Gondwana and late Cenozoic Hikurangi subduction margins. The results provide the foundation for future studies to investigate the kinematics and mechanism of the crustal exhumation and deformation of the North Island basement in further detail.</p>

Thermo-tectonic studies of Mesozoic basement rocks, North Island, New Zealand

<p>The basement rocks of North Island, New Zealand, comprise metasedimentary terranes that were accreted onto the eastern Gondwana margin during Mesozoic subduction. Since the Oligocene, these terranes have been sitting at the leading edge of the Australian Plate, as the hanging wall of the Hikurangi subduction margin, overriding the subducting Pacific Plate. This thesis examines the thermo-tectonic histories of the basement rocks in North Island, using fission-track and (U-Th-Sm)/He thermochronology. In eastern North Island, thermochronological data from the basement rocks record the exhumation histories since the latest Jurassic, related to two subduction cycles. Zircon fission-track analysis yields detrital or slightly reset ages (264–102 Ma); apatite fission-track ages range from 122 to 7.9 Ma and (U-Th-Sm)/He from 33.3 to 6.0 Ma. In central North Island, modelled thermal histories suggest that the basement rocks were exhumed to shallow levels (<2 km) of the crust in the Early Cretaceous (~150–135 Ma). This was followed by a period of reheating until ~100 Ma, which is interpreted to be the result of burial by sedimentation above the accretionary wedge. From 100 Ma, models indicate thermo-tectonic quiescence until the Late Oligocene. During the late Cenozoic, exhumation of the basement rocks accelerated at ~27 Ma in the western margin of the axial ranges (Kaimanawa Mountains). This acceleration in exhumation rate is interpreted to reflect the initiation of the subduction of the Pacific Plate beneath central North Island. Since the Late Oligocene, basement exhumation in the axial ranges migrated towards the trough. Modelled thermal histories indicate significant eastwards reverse faulting on the margin-parallel Ngamatea Fault between ~27 and 20 Ma and on the Wellington-Mohaka Fault between ~20 and 10 Ma. In contrast to the activity in the axial ranges, in western North Island, the exhumational response of the basement rocks to the Cenozoic subduction was less significant and not revealed from the present thermochronological data. Since the Late Miocene, the exhumation rate in the axial ranges has varied significantly along-strike, lower in the centre and higher to the north and south. During the last 10 Myr, the total magnitude of exhumation has been ~4 km in the Wellington region in the south, >1 km in the Raukumara Range in the north and negligible (less than a few hundred metres) in the central axial ranges in the Hawke’s Bay region. Although the accumulation of underplated material at the basal upper plate may have contributed to the localised rock uplift and exhumation (e.g. in the Raukumara Range), margin-normal shortening of the upper plate in the forearc of the Hikurangi Margin has most likely dominated the unroofing process of the axial ranges. In northwestern North Island, the Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto in situ Mesozoic and early Cenozoic rocks. Detrital zircon and apatite fission-track ages reveal that the basal Northland Allochthon sequences and the underlying Miocene autochthonous sedimentary rocks were predominantly derived from the local Jurassic terrane (Waipapa Supergrop) and perhaps the Late Cretaceous volcanics. In addition, the Early Miocene autochthon contains significant sedimentary influx from the Late Oligocene volcanics related to the subduction initiation in northern New Zealand. Zircon and apatite fission-track data from the in situ Mesozoic basement were inverted using thermo-kinematic models coupled with an inversion algorithm. The results suggest that during the Late Oligocene, ~4–6 km thick nappes were emplaced onto the in situ rocks in the northernmost Northland region. Prior to basement unroofing in the Early Miocene, the nappes thinned towards the south. Following allochthon emplacement, eastern Northland was uplifted and unroofed rapidly over a period of ~1–6 Myr, leading to ~0.4–1.5 km erosion of the allochthon. Since the mid-Miocene, due to the decline in tectonic activity in this region, the Northland Allochthon and the underlying rocks have been eroded slowly. This thesis has documented variable exhumation and burial processes that occurred in the upper plates of both the Mesozoic Gondwana and late Cenozoic Hikurangi subduction margins. The results provide the foundation for future studies to investigate the kinematics and mechanism of the crustal exhumation and deformation of the North Island basement in further detail.</p>

Sequence Stratigraphy, Chronostratigraphy and Zircon Geochronology of the CIROS-1 Drill Core, Ross Sea, Antarctica: Implications for Cenozoic Glacial and Tectonic Evolution

<p>Antarctica plays a central role in the global climate system. Understanding the continent's past climate interactions is key to predicting its future response to, and influence on, global climate change. In recent decades, sediment cores drilled on the Antarctic continental margin have provided direct evidence of past climatic and tectonic events. Drilled in 1986 from sea ice in western McMurdo Sound, the pioneering 702 m-long CIROS-1 core extended back to the Late Eocene and provided some of the first evidence of the antiquity and history of the Antarctic ice sheets. The CIROS-1 drill core recovered a depositional history of the western margin of the Victoria Land Basin adjacent to the Trans-Antarctic Mountains. It was located directly offshore from where the Ferrar Glacier, which drains the East Antarctic Ice Sheet, discharges into the Ross Sea. Consequently CIROS-1 contains a record of both the glacial and tectonic Cenozoic evolution of the Antarctic margin. This thesis provides a timely re-evaluation of the CIROS-1 core with new analysis techniques that enable further insights into the glacial and tectonic history of the western Ross Sea region, and includes three key objectives: (1) Re-examine CIROS-1 sedimentology and stratigraphy and provide a new facies and sequence stratigraphic analysis using modern methods developed from recent drilling projects (e.g. CRP, ANDRILL). (2) Develop a new integrated chronostratigraphic model through an assessment and compilation of previous studies, which provides a context for the interpretation of detrital zircon data, climate and tectonic history. (3) Undertake a detailed examination of the provenance of CIROS-1 sediments using cutting edge in situ analysis techniques of detrital zircons (U-Pb and trace element analysis using LA-ICP-MS). Glaciomarine sequence stratigraphic analysis identifies 14 unconformity-bound sequences occurring in two distinctive stratigraphic motifs. The four sequences located beneath the 342 mbsf unconformity contain relatively complete vertical facies succession. They were deposited in shallow marine, fluvio-deltaic conditions with distal glaciers terminating on land, and possibly calving into the ocean in adjacent valleys as evidenced by occasional ice-rafted debris. The ten sequences located above ~342 mbsf have a fundamentally different architecture. They are incomplete (top-truncated), contain subglacial and ice proximal facies grading upsequence into distal glaciomarine and shelf conditions. Top truncation of these sequences represents overriding of the CIROS-1 site by the paleo-Ferrar Glacier during glacial phases. A revised age model for CIROS-1 is presented that utilises new calibrations for Antarctic diatom zones and compiles three previously published age models for different sections of the core (Roberts et al., 2003; Wilson et al., 1998; Hannah et al., 1997). The new age model allows correlation of Late Oligocene cycles with coeval cycles in CRP-2/2A, 80 km to the north. A fundamental orbital control on the dynamics of these East Antarctic Ice Sheet outlet glaciers is evident from this comparison. Both glacier systems respond in-phase to longer-period orbital components (e.g. eccentricity 100 kyr and 400 kyr), but differ in their sensitivity to precession (20 kyr). It appears that during the Late Oligocene the Ferrar catchment responded to 20 kyr precession cycles, whilst the larger MacKay Glacier, which is more directly connected to the East Antarctic Ice Sheet, responded to longer duration 125 kyr (eccentricity) forcing. CIROS-1 zircons group into four distinct geochemical suites. Zircons formed in felsic igneous environments dominate the CIROS-1 population, with 89 % of zircons analysed showing geochemical characteristics inherent to granitic/rhyolitic zircons. Approximately 7 % of CIROS-1 zircons have a highly trace element enriched igneous provenance and were most probably sourced from enriched enclaves in granitic/rhyolitic units or from pegmatites. Approximately 3 % of CIROS-1 zircons show a metamorphic geochemical signature, and ~1 % formed in trace element depleted igneous environments. The zircons were sourced from the local basement (Koettlitz, Granite Harbour Groups), the Beacon Supergroup, and potentially, lithologies of the East Antarctic Craton located under the ice, or components of the Trans-Antarctic Mountains located under the current baseline of geologic exposure. Large-scale, systematic temporal trends in zircon characteristics have been divided into three distinct climatic periods: Zone 1 (702-366 mbsf, Late Eocene), Zone 2 (366-250 mbsf, Late Oligocene) and Zone 3 (< 250 mbsf, Late Oligocene and Early Miocene). Zircons deposited during these periods show unique properties. During Zone 1, Antarctica experienced a relatively warm temperate climate and alpine style glaciers flowed eastwards through the Trans-Antarctic Mountains. Zircons in this zone contain a subtle record of unroofing of geochemically zoned Granite Harbour and Koettlitz units located in the Ferrar Valley. During Zone 2 deposition, glaciers flowed though the Trans-Antarctic Mountains draining a large and ephemeral EAIS, which oscillated on orbital time scales. Zircons in this interval show variable properties, high numbers and were most probably deposited as the paleo-Ferrar Glacier deeply incised the Ferrar Fiord. In contrast, Zone 3 is characterised by a flux of McMurdo Volcanic Complex derived sediments, together with systematic changes in zircon characteristics. These patterns indicate a Late Oligocene shift in ice flow to the site (above ~250 mbsf). Due to a cooling that culminated in the Mi-1 glaciation, ice flow to the site changed from an eastward to a northward flow, in response to an increased ice volume in the Ross embayment.</p>