A marine to freshwater sediment succession from Kowhai Beach wetland, Northland: Implications for Holocene sea level

Abstract

An infilled wetland located behind coastal dunes in north-east Northland is used to reconstruct a local history of environmental change spanning early Holocene (c. 7000 yr BP) to modern time. Proxy indicators (sediment texture, diatoms and pollen) provide evidence for a transition from marginal marine-to brackish-to freshwaterconditions in the wetland. Radiocarbon ages constrain the chronology of this succession to 7880-7430 cal. yr BP for the early period of marine conditions, 3570-3210 cal. yr BP for the latter brackish phase and 1060-800 cal. yr BP for the change to freshwater conditions. Within this succession, the diatom record preserves a strong brackish signal at core depths above the limit of the modern tidal range. This is presented as preliminary evidence for a mid-Holocene sea level highstand for northern New Zealand of approximately 1.2 m above present mean sea level. Keywords diatoms;14 C dating; highstand; palynology; sea level; sediments; stratigraphy INTRODUCTIONThe characteristics of sedimentary successions that constitute the coastal depositional record are controlled by the interplay between sea level, sediment supply and accommodation space. Studies of the facies that comprise these sedimentary successions traditionally focus on shoreface, beach and dune environments and associated marine-dominated processes (e.g., Curray 1964;Kraft 1971; Thorn 1983;Davis & Clifton 1987;Roy et al. 1994). In general, these particular environments do not always preserve a detailed record of environmental change in adjacent terrestrial systems. In contrast, back-barrier environments (wetlands, swamps, marshes and lagoons) do have the potential to record details of environmental change in both coastal and terrestrial systems, particularly responses to sea-level change .For the New Zealand coast, there has been little effort to use back-barrier environments to study environmental change, nor to resolve Holocene sea-level history beyond that established by Gibb (1986). Noticeably absent has been any attempt to establish evidence for a midHolocene highstand in sea level, a phenomenon now well documented for other coasts in the SW Pacific (e.g., Woodroffe et al. 1995;Baker & Haworth 1997Nunn 1998; Sloss et al. in press). This paper addresses these matters via a case study of a back-barrier wetland in northern New Zealand. The study objectives are to investigate the sensitivity of the wetland to marine and terrestrial influences, and to test for evidence of a sea-level highstand using a multiproxy analysis of the sediment record.

Investigating subduction earthquake geology along the southern Hikurangi margin using palaeoenvironmental histories of intertidal inlets

“…However, a recent review of New Zealand SL data by Clement et al (2008Clement et al ( , 2010 suggests that SL stabilised at 7550 cal yr BP and was 0.5Á1 m higher than modern mean SL from 7550 to 2000 cal yr BP. Further studies also suggest there may have been a 1.2Á1.5 m midHolocene highstand in New Zealand (Schofield 1960;Hicks & Nichol 2007;Kennedy 2008), consistent with other studies from the southwest Pacific (e.g. Sloss et al 2007;Switzer et al 2010).…”

Preservation of a Late Glacial terrestrial and Holocene estuarine record on the margins of Kaipara Harbour, Northland, New Zealand

“…In doing so, we assume that the North Kaipara barrier has been tectonically stable during this time, experiencing neither major subsidence nor uplift. This assumption is supported by morphostratigraphic evidence at several sites on the Northland coast, including Kaipara Harbour, that tie the mid-Holocene sea level highstand to a common elevation of +1.2 to 2.1 m (Richardson 1985;Hicks & Nichol 2007), a range that is consistent with other tectonically stable sites in the SW Pacific (e.g., Woodroffe et al 1995;Sloss et al 2007).…”

An examination of spatial variability in the timing and magnitude of Holocene relative sea-level changes in the New Zealand archipelago

“…While the sea-level history presented by Gibb (1986) has become the (de facto) benchmark or 535 standard reconstruction of Holocene sea-level change for New Zealand, it contrasts with much 536 recent research from the wider southwest Pacific region (Hicks and Nichol, 2007;Clement, 2011). 537 23 For example, while many studies from the southwest Pacific have documented higher than present 538 mid-Holocene sea levels (e.g., Nunn, 1995Nunn, , 1998 Woodroffe et al, 1995; Baker and Haworth, 1997, 539 2000a, b; Baker et al, 2001a, b; Horton et al, 2007;Sloss et al, 2007; Woodroffe, 2009; Lewis et al, 540 2013), New Zealand workers have been noticeably reticent on the subject (Hicks and Nichol, 2007).…”

“…537 For example, while many studies from the southwest Pacific have documented higher than present 538 mid-Holocene sea levels (e.g., Nunn, 1995Nunn, , 1998 2013), New Zealand workers have been noticeably reticent on the subject (Hicks and Nichol, 2007). 541 Kennedy (2008) noted that sea-level histories from southeast Australia and the Tasman Sea in 542 particular may be significant in the New Zealand context, as these records come from within the 543 same oceanic region as New Zealand (e.g., Clark and Lingle, 1979;Pirazzoli, 1991), and so may be 544 expected to yield similar histories of Holocene sea-level fluctuations.…”

“…These deformation rates were then used to adjust the observed 85 elevations of these same indicators for tectonic movement, supposedly yielding a RSL signal 86 unaffected by tectonic deformation. This is a circular argument, and the base assumption of stable 87Late Holocene sea level no longer holds, as a mid-Holocene sea-level highstand is indicated by a 88 large number of studies of Holocene RSL change in New Zealand, including glacial-isostatic 89 adjustment (GIA) models of Holocene sea-level change in New Zealand (e.g., Peltier, 1988; Nakada 90 and Lambeck, 1989; Gehrels et al, 2012), geomorphic studies from a number of New Zealand 91 locations (e.g., Hull, 1985;Hicks and Nichol, 2007;Kennedy, 2008;Schallenberg et al, 2012), and 92 other New Zealand Holocene sea-level reconstructions (e.g., Hayward et al, 2010a, b, c; Clement 93 2011). In the wider context of the southwest Pacific a large number of studies show a mid-Holocene 94 sea-level highstand (e.g., Nunn, 1995Nunn, , 1998 Woodroffe et al, 1995; Baker and Haworth, 1997, 95 2000a, b; Baker et al, 2001a, b; Woodroffe, 2009; Lewis et al, 2013).…”