Ashwaq T. Sabaa

Effect and timing of increased freshwater runoff into sheltered harbor environments around Auckland City, New Zeland

Two short cores of late Holocene, low tidal, estuarine sediment from the sheltered fringes of the Aucklands Waitemata Harbor, New Zealand, record the following changes through time since human colonization: an abrupt decline and disappearance of marine molluscs, a major decline and virtual disappearance of ostracods, an abrupt decline in calcareous foraminifera (mostlyAmmonia spp.), a rapid increase, in abundance of agglutinated foraminifera, large diatoms, and freshwater thecamoebians, and an increase in sedimentation rate, but no consistent trend in change of grain size. The up-core foraminiferal changes mimic their present day up-estuary zonation, which correlates strongly with decreasing salinity and pH. In both localities the faunal changes can be correlated with the documented local land-use history and increased freshwater runoff over time. At the head of the Waitemata Harbor, in Lucas Creek estuary, three phases of foraminiferal faunal change occurred: minor changes during initial Polynesian forest clearance (1500–1800 AD), a major change in early European times (1840–1870 AD) with clearance of most of the remaining native forest, and another small change in very, recent times (∼1990s) with urbanization in the Lucas Creek catchment. On the eastern, seaward fringes of the Waitemata Harbor, in the smaller Tamaki Estuary, no faunal changes occurred in association with complete forest clearance and establishment of pastoral farming in Polynesian and early European times (before 1950s). Major foraminiferal and other faunal changes occurred in the late European period (1960s–1970s) coincident with the onset of major urbanization spreading throughout the Tamaki catchment. Our results suggest increased freshwater runoff is the major culprit for many of the observed biotic changes in the urbanized estuaries of New Zealand.

Microfossil record of the Holocene evolution of coastal wetlands in a tectonically active region of New Zealand

The shallow tidal Wairau coastal lagoons, New Zealand, are in a prime location for investigating the relative roles of tectonic and eustatic sea level on their palaeogeographic evolution. The Wairau lagoons are unique in New Zealand for their wide seasonal and tidal salinity range, from hyposaline (10—20 psu) to hypersaline (35—54 psu). Foraminiferal and ostracod associations are recognised, using Q-mode cluster analysis, living in and around these lagoons and detrended canonical correspondence analysis (DCCA) shows that their distributions are strongly correlated with tidal elevation and salinity. Analyses of the modern analogue faunal data combined with Holocene microfaunal data from five 2.5—9 m deep cores enables direct palaeoenvironmental interpretation of the fossil faunas and elucidation of the lagoons’ palaeogeographic evolution. The area was inundated by rising eustatic sea level from 8.5 ka onwards, forming a fully marine, sheltered, subtidal bay. Sediment supply outpaced local tectonic subsidence and the bay filled with mud, shallowing to intertidal by 4.5—3.5 ka, still with an open mouth to the sea. Since then sediment supply has kept pace with 3—4 m of inferred tectonic subsidence. At ~1.5 ka the calcareous-dominated foraminiferal faunas suddenly changed to agglutinate-dominated faunas, indicating a switch to a semi-closed lagoon linked to the Wairau River estuary, with highly varied salinity like today. We infer this was caused by northwards extension of the Wairau Boulder Bank across the bay’s mouth in response to a sharp eustatic sea-level fall after 2 ka. Sediment supply switched to fluvially derived sand which built a flood-delta into the lagoon dividing it into three water bodies. Relative sea-level rise in the last 600 years from earthquake-related compaction (AD 1855) and accelerating eustatic rise (0.6 m) has resulted in increased marginal erosion of the lagoons and their re-amalgamation into one linked water body.

Recent benthic foraminifera from offshore Taranaki, New Zealand

Abstract Paleobathymetric estimates based on fossil foraminiferal faunas play an important role in understanding the paleogeographic, structural, and burial history of New Zealands most important hydrocarbon‐bearing sedimentary basin—the Taranaki Basin. Bathyal and abyssal estimates have large ranges of uncertainty, which might be improved using knowledge of the depth distribution patterns of Recent benthic foraminifera in the same region. Four benthic foraminiferal groups (and 9–10 associations) are recognised and mapped in the offshore Taranaki region (0–2150 m depth, eastern Tasman Sea), based on two separate cluster analyses of census data (231 species, 39 samples) on faunas with tests >63 and >150 μm. The same depth pattern can be identified using 63 or 150 μm faunas, although there are major differences in the dominant taxa. Canonical correspondence analysis and correlation coefficients suggest that the distribution patterns are strongly depth related: (1) inner shelf (0–50 m) associations (both shell‐size fractions) are dominated by Rosalina irregularis and Zeaflorilus parri; (2) outer shelf‐uppermost bathyal (50–550 m) associations are dominated by Bulimina marginata s.s. and Discorbinella bertheloti (both sizes) plus Cassidulina carinata (>63 μm) or Cibicides dispars (>150 μm); (3) middle‐lower bathyal (500–1500 m) associations are dominated by C. carinata‐Alabaminella weddellensis‐Abditodentrixpseudothalmanni (>63 μm) and Uvigerina peregrina‐Bulimina marginata f. aculeata (>150 μm); and (4) lower bathyal to upper abyssal (1400–2150 m) associations are dominated by B. marginata f. aculeata and Globocassidulina subglobosa (both) plus A. weddellensis (>63 μm) or U. peregrina‐Oridorsalis umbonatus (>150 μm). Comparison of the >63 μm Taranaki (west coast) faunal data with a similar dataset from east of New Zealand shows significant differences in composition, relative abundance levels, and depth ranges of common species, which appear to be a result of differences in primary productivity, translated into organic carbon flux (food). Since organic carbon flux reaching the seafloor decreases progressively with increasing water depth, we infer that this is the major factor producing the strong depth‐related distributional pattern of deep‐sea benthic foraminiferal faunas observed around New Zealand. Thus, highly accurate estimates of paleobathymetry are unlikely using benthic foraminifera, unless organic carbon flux has remained unchanged. Notwithstanding the differences between the west and east coasts, there are sufficient similarities and trends that are bathymetrically consistent to be useful in improving paleobathymetric estimates. These include, in decreasing order of reliability: upper depth limits of key benthic species; recognition of benthic foraminiferal associations; and relative abundance of planktic foraminifera. Species diversity measures show no useful pattern with depth.

Changes in the position of the Subtropical Front south of New Zealand since the last glacial period

This study fills an important gap in our understanding of past changes in the Southern Subtropical Front (S-STF) in the southwest Pacific Ocean. Paleo-sea surface temperatures (SST) were estimated from planktic foraminiferal census counts from cores straddling the modern S-STF in the Solander Trough, south of New Zealand. The estimated SST were compared for 6 time slices; glacial period (25-21 ka), Last Glacial Maximum (LGM; 21-18 ka), early deglaciation (18-16 ka), late deglacial/early Holocene period (14-8 ka), mid-Holocene period (8-4 ka), and late Holocene period (4-0 ka). The position of the S-STF was determined by two methods: (1) the location of the 10 degrees C isotherm and (2) the location of the highest SST gradients. These new results suggest that the S-STF was not continuous between east and west of New Zealand during the glacial period. Steep SST gradients indicate that a strong S-STF rapidly shifted south during the LGM and early deglaciation. During the late deglacial and Holocene periods the position of the S-STF differs between the two methods with reduced SST gradients, suggesting amore diffuse S-STF in the Solander Trough at this time. The glacial SST data suggest that the S-STF shifted north to the west of New Zealand, while to the east there was a stronger SST gradient across the front. This was possibly the result of an increased wind stress curl, which could have been caused by stronger, or more northerly Southern Hemisphere westerly winds (SHWW), or a merging of the SHWW split jet in this region.

Pliocene sea surface temperature changes in ODP Site 1125, Chatham Rise, east of New Zealand

Abstract Planktonic foraminiferal census counts were converted to sea surface temperature (SST) estimates using the modern analogue technique (MAT) for the middle–late Pliocene (4.0–2.37 Ma) in ODP Site 1125, north side of Chatham Rise, SW Pacific Ocean. MAT SSTwarm records range between 8°C and 20.5°C, and MAT SSTcold records parallel that pattern but with a temperature range of 5–15°C. The modern position of Site 1125 is just north of the Subtropical Front and has an annual temperature range of ∼14–18°C. Pliocene warmest temperatures are 1–2° warmer than modern summers, whereas cold season SST records are up to 6–10°C cooler than modern winters. Overall average temperatures at the site are 2–3°C cooler than modern temperatures during a time of sustained global warmth. Three major cold excursions centred on 3.35, 3.0, and 2.8 Ma showed warm season temperatures over 5°C colder than the last glacial maximum, experiencing temperatures typical of modern subantarctic waters. Two minor cold excursions at 2.7 Ma and 2.4 Ma experienced temperatures cooler than modern winters but not as cold as last glacial conditions. Cold season SSTs show a shift to warmer climate upward through the study interval, whereas warm season estimates remain essentially unchanged. We interpret the strong regional cooling of subtropical Southwest Pacific water through the middle–late Pliocene as having been caused by increased upwelling. It is also possible that the subtropical frontal zone moved north over the site in the Pliocene, however, this is considered the least likely interpretation. Our record of cool conditions in the Southwest Pacific corroborate evidence of cooler than modern conditions in other regions of the western Pacific through the mid-Pliocene despite overall global warming.

A foraminiferal proxy record of 20th century sea-level rise in the Manukau Harbour, New Zealand

The present study aimed to extract a sea-level history from northern New Zealand salt-marsh sediments using a foraminiferal proxy, and to extend beyond the longest nearby tide-gauge record. Transects through high-tidal salt marsh at Puhinui, Manukau Harbour, Auckland, New Zealand, indicate a zonation of dominant foraminifera in the following order (with increasing elevation): Ammonia spp.–Elphidium excavatum, Ammotium fragile, Miliammina fusca, Haplophragmoides wilberti–Trochammina inflata, Trochamminita salsa–Miliammina obliqua. The transect sample faunas are used as a training set to generate a transfer function for estimating past tidal elevations in two short cores nearby. Heavy metal, 210Pb and 137Cs isotope analyses provide age models that indicate 35 cm of sediment accumulation since ~1890 AD. The first proxy-based 20th century rates of sea-level rise from New Zealand’s North Island at 0.28 ± 0.05 cm year–1 and 0.33 ± 0.07 cm year–1 are estimated. These are faster than the nearby Auckland tide gauge for the same interval (0.17 ± 0.1 cm year–1), but comparable to a similar proxy record from southern New Zealand (0.28 ± 0.05 cm year–1) and to satellite-based observations of global sea-level rise since 1993 (0.31 ± 0.07 cm year–1).

Marine submersion of an archaic moa-hunter occupational site, Shag River estuary, North Otago

Abstract At Shag River estuary, North Otago, New Zealand, parts of an Archaic moa-hunter occupation site (AD 1340–1410) lie beneath 50 cm of saltmarsh sediment at a modern elevation of 0.3 m below mean high water. Fossil saltmarsh foraminiferal assemblages in the overlying cored sediment provide high-tidal palaeo-elevation estimates (Modern Analogue Technique) that indicate a gradual rise in relative sea level of 0.59±0.05 m since that time. This part of the moa-hunter site appears to have been located on an unvegetated high-tidal sand flat at the time of occupation. There is no evidence that supports previously hypothesised coseismic compaction of underlying sediment as a cause for the submergence during the time of occupation. Comparison with other saltmarsh foraminiferal records in southern New Zealand suggests that the moa-hunter occupation occurred during the interval of lowest sea level in the last millennium (early part of the Little Ice Age) and that approximately half the subsequent sea-level rise occurred after AD 1900.

Biogeography and ecological distribution of shallow-water benthic foraminifera from the Auckland and Campbell Islands, subantarctic southwest Pacific

One hundred and forty-eight species of benthic foraminifera are recorded from depths shallower than 80 m around the subantarctic Auckland (130 spp.) and Campbell (71 spp.) Islands, southwest Pacific. Comparisons with other circum-polar, subantarctic island groups suggest that they all have relatively low diversity, shallow-water benthic, foraminiferal faunas, with their sheltered harbours dominated by species of Elphidium, Notorotalia, Cassidulina, Haynesina and Nonionella-Nonionellina. More exposed environments are dominated by a small number of species of Cibicides, Miliolinella, Rosalina, Quinqueloculina and Glabratellidae. The extremely low species richness (three species) in high-tidal grass-dominated salt marsh on Campbell Island is similar to that reported from Tierra del Fuego at a similar latitude. The faunas of Auckland and Campbell Islands have their strongest affinities (70–75% species in common) with New Zealand’s three main islands, 460–700 km away. Ten percent of their fauna has not been recorded from mainland New Zealand, reflecting one endemic species and a small element of apparently subantarctic and bipolar-restricted species. Since there have been no shallow-water (<500 m) links to other lands since these two Miocene volcanic islands were formed, it is concluded that most benthic foraminiferal species have arrived in suspension in eddies of surface water, many since the peak of the Last Glacial.

Salt-marsh foraminiferal record of 10 large Holocene (last 7500 yr) earthquakes on a subducting plate margin, Hawkes Bay, New Zealand

Sudden changes in microfossils and lithologies in Holocene sediments of a former tidal inlet on the Hikurangi subduction margin provide evidence of 10 large earthquakes. Studies were focused in three former embayments where intertidal shelly sediment interfingers with freshwater and salt-marsh peat. Paleoelevation histories were reconstructed using the modern analogue technique with foraminiferal assemblages. Land elevation record analysis indicates 8−9 m of mid- to late Holocene tectonic subsidence occurred prior to 1.5 m of uplift during the A.D. 1931 Hawkes Bay earthquake. Chronologies of displacement events were constrained using 50 radiocarbon dates and three widespread air-fall tephras. We infer the following earthquakes: earthquake 1: 7.3−7.0 ka (−1.1 ± 0.3 m), earthquake 2: 5.6−5.1 ka (+0.4 ± 0.4 m), earthquake 3: 5.2−4.9 ka (−0.5 ± 0.5 m), earthquake 4: 4.4−3.8 ka (−0.6 ± 0.5 m), earthquake 5: 2.8−2.4 ka (−0.9 ± 0.5 m), earthquake 6: 1.73−1.70 ka (−1.0 ± 0.3 m), earthquake 7: 1.5−1.3 ka (−0.7 ± 0.5 m), earthquake 8: 1.04−0.89 ka (−1.2 ± 0.4 m), earthquake 9: 0.60−0.44 ka (−0.8 ± 0.6 m), and earthquake 10: A.D. 1931 (+1.5 ± 0.3 m). A further 1.6−2.6 m of subsidence could have occurred by gradual aseismic slip or in smaller earthquakes. The age ranges of four of the recognized earthquakes (earthquakes 1, 6, 8, and 9) overlap with other documented displacement events onshore along 250−600 km of the Hikurangi subduction margin, and with turbidites offshore 100−300 km to the north. These four are considered strong candidates for large subduction-interface earthquakes. The other five inferred earthquakes are less strongly correlated with along-margin displacement events and offshore turbidites. These could have been caused by upper-plate fault ruptures (like historic earthquake 10), but subduction-interface sources cannot be ruled out. This evidence for repeated coseismic vertical deformation suggests large coseismic slip on a part of the subduction interface beneath Hawkes Bay that is currently dominated by aseismic creep processes, such as transient slow-slip events. This clearly indicates multiple slip processes are possible in a single location on a subduction interface.

Foraminiferal record of Holocene paleo-earthquakes on the subsiding south-western Poverty Bay coastline, New Zealand

Foraminiferal faunas in 29 short cores (maximum depth 7 m) of estuarine and coastal wetland sediment were used to reconstruct the middle–late Holocene (last 7 ka) elevational history on the southern shores of Poverty Bay, North Island, New Zealand. This coast is on the southwest side of a rapidly subsiding area beneath western Poverty Bay. Modern Analogue Technique paleo-elevation estimates based on fossil foraminiferal faunas indicate that the four study areas have gradual late Holocene (<3.5 ka) subsidence rates that increase from the southwest (mean c. 0.5 m ka–1) to northeast (mean c. 1.0 m ka–1). Only two rapid, possibly co-seismic, vertical displacement events are recognised: (1) c. 1.2 m of subsidence at 5.7 ± 0.4 ka (cal yr BP), which may have been generated by a subduction interface earthquake centred offshore and recorded in other published studies in northern Hawkes Bay, c. 35 km to the south; and (2) c. 1 m of uplift (relative sea-level fall) at c. 4.5 ± 0.3 ka, which might have been generated by rupture on an offshore upper plate fault that also uplifted coastal terraces at Pakarae and Mahia, 40 km to the north and south of the study area, or by rupture on the subduction interface penetrating beneath Poverty Bay. No sudden displacement events are recognised during the last 4 ka although subsidence, possibly aseismic, has continued.