title: Tubular Carbonate Concretions from North Island, New Zealand: Evidence for Hydrocarbon Migration and the Subsurface Plumbing System of Cold Seeps

authors: Nyman, Stephanie Leigh

subjects: hydrocarbon migration

abstract: Among the cold seep research community, it is now appreciated that tubular carbonate concretions are important indicators of hydrocarbon migration in the subsurface. In the last few years, several publications have documented tubular concretions and interpreted a subsurface seep origin. However, the nature, timing, and relative importance of tubular concretions as records of fluid flux and chemistry over time is yet to be fully appreciated.

In North Island, New Zealand, tubular concretions are geographically widespread but occur in localised clusters in several Cenozoic (especially Miocene) sedimentary formations of the East Coast Basin (Hikurangi Margin) and Taranaki Basin. They formed by precipitation of micritic dolomite and calcite cement within host siliciclastic mudstone and contain 50 to 85% carbonate (dolomite dominated), indicating precipitation in shallow burial (0 to 300 m). Several concretion morphologies occur, especially pipe, bulbous, doughnut, corkscrew, and conical shapes, and they include some of the largest examples known worldwide, ranging up to 10 m or more in length (limited only by exposure) and 0.1 to 1 m in diameter. The concretions typically support near-central conduits, from 1 to 40 cm in diameter, which may be open or variably filled with sediment and/or late cements.

As well as diverse morphologic types, the tubular concretions also show, within limits, variable mineralogic, petrographic, and geochemical characteristics. Additionally, some examples display association with slope instability, fault control on fluid migration, and stratigraphic placement directly below ancient seafloor seep carbonates. δ13C values of the cement forming the concretions range from -52 to +13 PDB and are interpreted to reflect carbonate precipitation from the onset of methane migration to the end of a major fluid migration event. A trend from strongly negative to strongly positive δ13C values reflects either a mixing of methane and methanogenic CO2 and/or the extensive anaerobic oxidation of methane (AOM) as supported by lipid biomarkers. δ18O values range from −3 to +5 PDB suggesting an evolved fluid source influenced by cycles of methane hydrate formation and dissociation.

The tubular carbonate concretions are interpreted to represent the subsurface plumbing pathways of methane expulsion in ancient hydrocarbon seep systems in North Island. Additionally, they suggest that gas hydrates may have been forming and dissociating along the Hikurangi Margin off eastern North Island for the past 23 Ma. The diverse geologic characteristics of the tubular concretions provide a unique opportunity to construct a comprehensive 4-D model of the subsurface development of such a system. The resulting schematic model of tubular concretion formation is an analogue for the subsurface fluid migration system of hydrocarbon seeps along the modern Hikurangi Margin, and possibly for many modern and ancient hydrocarbon seep systems in general.

text: Tubular Carbonate Concretions from North Island, New Zealand: Evidence for Hydrocarbon Migration and the Subsurface Plumbing System of Cold Seeps Among the cold seep research community, it is now appreciated that tubular carbonate concretions are important indicators of hydrocarbon migration in the subsurface. In the last few years, several publications have documented tubular concretions and interpreted a subsurface seep origin. However, the nature, timing, and relative importance of tubular concretions as records of fluid flux and chemistry over time is yet to be fully appreciated.

In North Island, New Zealand, tubular concretions are geographically widespread but occur in localised clusters in several Cenozoic (especially Miocene) sedimentary formations of the East Coast Basin (Hikurangi Margin) and Taranaki Basin. They formed by precipitation of micritic dolomite and calcite cement within host siliciclastic mudstone and contain 50 to 85% carbonate (dolomite dominated), indicating precipitation in shallow burial (0 to 300 m). Several concretion morphologies occur, especially pipe, bulbous, doughnut, corkscrew, and conical shapes, and they include some of the largest examples known worldwide, ranging up to 10 m or more in length (limited only by exposure) and 0.1 to 1 m in diameter. The concretions typically support near-central conduits, from 1 to 40 cm in diameter, which may be open or variably filled with sediment and/or late cements.

As well as diverse morphologic types, the tubular concretions also show, within limits, variable mineralogic, petrographic, and geochemical characteristics. Additionally, some examples display association with slope instability, fault control on fluid migration, and stratigraphic placement directly below ancient seafloor seep carbonates. δ13C values of the cement forming the concretions range from -52 to +13 PDB and are interpreted to reflect carbonate precipitation from the onset of methane migration to the end of a major fluid migration event. A trend from strongly negative to strongly positive δ13C values reflects either a mixing of methane and methanogenic CO2 and/or the extensive anaerobic oxidation of methane (AOM) as supported by lipid biomarkers. δ18O values range from −3 to +5 PDB suggesting an evolved fluid source influenced by cycles of methane hydrate formation and dissociation.

The tubular carbonate concretions are interpreted to represent the subsurface plumbing pathways of methane expulsion in ancient hydrocarbon seep systems in North Island. Additionally, they suggest that gas hydrates may have been forming and dissociating along the Hikurangi Margin off eastern North Island for the past 23 Ma. The diverse geologic characteristics of the tubular concretions provide a unique opportunity to construct a comprehensive 4-D model of the subsurface development of such a system. The resulting schematic model of tubular concretion formation is an analogue for the subsurface fluid migration system of hydrocarbon seeps along the modern Hikurangi Margin, and possibly for many modern and ancient hydrocarbon seep systems in general.

year: 2009

title: Changing perceptions about feminists and (still not) claiming a feminist identity

authors: Dyer, Suzette L.

subjects: Social Sciences

abstract: We examine student perceptions about feminists and feminism, and the willingness to claim a feminist identity and engage in collective activism, as stated at the beginning and end of a Women’s Studies course. Course participation simultaneously fostered more positive views towards feminists and feminism and entrenched the unwillingness to claim a feminist identity and engage in activism. These contradictory outcomes stemmed from the critical capacity to recognise that structural inequality is reproduced through disciplinary relationships. Thus, unwillingness was entangled with feelings of fear and vulnerability in relation to the national context whereby neoliberalism guides the governance of the self, and where gender equality has presumed to be achieved. The article highlights that developing the willingness to identify and act is intimately shaped and constrained by the socio-political context and personal relationships. We consider the implications of this insight in relation to pedagogical assumptions about developing feminist knowledge in the classroom.

text: Changing perceptions about feminists and (still not) claiming a feminist identity We examine student perceptions about feminists and feminism, and the willingness to claim a feminist identity and engage in collective activism, as stated at the beginning and end of a Women’s Studies course. Course participation simultaneously fostered more positive views towards feminists and feminism and entrenched the unwillingness to claim a feminist identity and engage in activism. These contradictory outcomes stemmed from the critical capacity to recognise that structural inequality is reproduced through disciplinary relationships. Thus, unwillingness was entangled with feelings of fear and vulnerability in relation to the national context whereby neoliberalism guides the governance of the self, and where gender equality has presumed to be achieved. The article highlights that developing the willingness to identify and act is intimately shaped and constrained by the socio-political context and personal relationships. We consider the implications of this insight in relation to pedagogical assumptions about developing feminist knowledge in the classroom.

year: 2018

title: Sharing, listening, learning and developing understandings of Kaupapa Māori research by engaging with two Māori communities involved in education.

authors: Kana, Fred

subjects: Kaupapa Maori research methodology

abstract: This paper is a culmination of common understandings that were elicited from two pieces of research: ‘The Impact of the BHP New Zealand Steel Mining on the Tangata Whenua and the Environment’ and ‘The Impact of Maori Medium Education within a Mainstream Secondary School on the Lives of its Participants, in particular the Teachers, Caregivers and Students’. It was at the conclusion of each research project and as a consequence of informal conversations and discussions that this paper evolved. The paper discusses shared understandings in the context of Kaupapa Māori research methodology and key findings from the two research projects.

text: Sharing, listening, learning and developing understandings of Kaupapa Māori research by engaging with two Māori communities involved in education. This paper is a culmination of common understandings that were elicited from two pieces of research: ‘The Impact of the BHP New Zealand Steel Mining on the Tangata Whenua and the Environment’ and ‘The Impact of Maori Medium Education within a Mainstream Secondary School on the Lives of its Participants, in particular the Teachers, Caregivers and Students’. It was at the conclusion of each research project and as a consequence of informal conversations and discussions that this paper evolved. The paper discusses shared understandings in the context of Kaupapa Māori research methodology and key findings from the two research projects.

year: 2006