Late-Holocene Climate Variability in Southern New Zealand: A reconstruction of regional climate from an annually laminated sediment sequence from Lake Ohau

<p>This research aims to improve understanding of synoptic climate systems influencing southern New Zealand and document changes in the intensity and frequency of these systems beyond the historical record by analyzing a 1,350-year annually laminated sediment sequence recovered from Lake Ohau, South Island, New Zealand (44.234°S, 169.854°E). Climatological patterns originating in both the tropics (El-Niño-Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO)) and in the Antarctic (Southern Annular Mode (SAM)) influence year-to-year variability in New Zealand’s climate (e.g. temperature and precipitation). However, the range of natural variability of these systems in the southwest Pacific over time is poorly known because the instrumental record is short (~100 years). The high-resolution record from Lake Ohau offers a unique opportunity to investigate changes in regional hydrology and climate, and to also explore connections to large-scale climate patterns over the last millennium. Hydrodynamic and hydroclimatic processes that influence and control the production, transport, and deposition of sediment within the Lake Ohau catchment are examined and constrained in order to develop a robust climate record. A key aim is to determine the role that meteorology and climate play in controlling sediment flux. The physical properties and facies of a 5.5-meter-long Lake Ohau sediment core are analyzed using thin-sections, high-resolution X-radiographs scans, and particle-size analyses. Time-series analysis is used to establish links between varve facies, hydroclimate variability and regional synoptic climate types over the instrumental record. Utilizing this climate-proxy relationship, inflow conditions are reconstructed over the last 1,350 years and compared with regional temperature reconstructions to generate a Western South Island paleo-atmospheric circulation index. Relationship between this paleocirculation index and other proxy reconstructions show significant variability in the relative forcing of tropical (ENSO) and Southern Hemisphere highlatitude (SAM) synoptic climate drivers on New Zealand and southwest Pacific climate. Overall, this work demonstrates that: a) the laminated sediments from Lake Ohau are varves and the formation of the annual stratigraphy is strongly controlled by lake hydrodynamics, in particular, thermal lake stratification; b) sediment stratigraphy reflects changes in austral warm period (December-May) inflow, enabling a highresolution reconstruction of hydroclimate over the last 1,350 years and; c) the generation of a paleocirculation index for the Western South Island points to significant changes between northerly or southerly dominated atmospheric conditions in southern New Zealand, particularly over the ‘Little Ice Age’ (1385-1710 AD). During this time, the strength of tropical teleconnections weakened and a strong negative phase SAM persisted. Comparison with high-resolution regional proxy records from Antarctica and the Central Pacific point to significant regional coherence with a strong negative phase SAM acting as a primary driver of the onset of Little Ice Age conditions across the South Pacific.</p>

Late-Holocene Climate Variability in Southern New Zealand: A reconstruction of regional climate from an annually laminated sediment sequence from Lake Ohau

<p>This research aims to improve understanding of synoptic climate systems influencing southern New Zealand and document changes in the intensity and frequency of these systems beyond the historical record by analyzing a 1,350-year annually laminated sediment sequence recovered from Lake Ohau, South Island, New Zealand (44.234°S, 169.854°E). Climatological patterns originating in both the tropics (El-Niño-Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO)) and in the Antarctic (Southern Annular Mode (SAM)) influence year-to-year variability in New Zealand’s climate (e.g. temperature and precipitation). However, the range of natural variability of these systems in the southwest Pacific over time is poorly known because the instrumental record is short (~100 years). The high-resolution record from Lake Ohau offers a unique opportunity to investigate changes in regional hydrology and climate, and to also explore connections to large-scale climate patterns over the last millennium. Hydrodynamic and hydroclimatic processes that influence and control the production, transport, and deposition of sediment within the Lake Ohau catchment are examined and constrained in order to develop a robust climate record. A key aim is to determine the role that meteorology and climate play in controlling sediment flux. The physical properties and facies of a 5.5-meter-long Lake Ohau sediment core are analyzed using thin-sections, high-resolution X-radiographs scans, and particle-size analyses. Time-series analysis is used to establish links between varve facies, hydroclimate variability and regional synoptic climate types over the instrumental record. Utilizing this climate-proxy relationship, inflow conditions are reconstructed over the last 1,350 years and compared with regional temperature reconstructions to generate a Western South Island paleo-atmospheric circulation index. Relationship between this paleocirculation index and other proxy reconstructions show significant variability in the relative forcing of tropical (ENSO) and Southern Hemisphere highlatitude (SAM) synoptic climate drivers on New Zealand and southwest Pacific climate. Overall, this work demonstrates that: a) the laminated sediments from Lake Ohau are varves and the formation of the annual stratigraphy is strongly controlled by lake hydrodynamics, in particular, thermal lake stratification; b) sediment stratigraphy reflects changes in austral warm period (December-May) inflow, enabling a highresolution reconstruction of hydroclimate over the last 1,350 years and; c) the generation of a paleocirculation index for the Western South Island points to significant changes between northerly or southerly dominated atmospheric conditions in southern New Zealand, particularly over the ‘Little Ice Age’ (1385-1710 AD). During this time, the strength of tropical teleconnections weakened and a strong negative phase SAM persisted. Comparison with high-resolution regional proxy records from Antarctica and the Central Pacific point to significant regional coherence with a strong negative phase SAM acting as a primary driver of the onset of Little Ice Age conditions across the South Pacific.</p>

Eolian sedimentation in central European Auel dry maar from 60 to 13 ka

The climate in central Europe during the last 60 ka is characterized by rapid temperature and moisture changes and strong cold periods (Heinrich events). All these variations are preserved in sediments of marine and also some terrestrial archives. Here we present a continuous, terrestrial sediment record with almost all Greenland stadials and Heinrich events between 60 and 13 ka visible from carbonate roundness of the Eifel Laminated Sediment Archive Dust Stack-20 and CaCO3 data for central Europe. The carbonate roundness data show almost all stadials between 60 and 13 ka. CaCO3 data show a general transport system change with the beginning of Heinrich event 3. Since there are no carbonates west of the Auel Maar, we conclude that the eolian-transported grains were not transported by westerly but easterly winds. These postulated easterly winds during the last glacial maximum are supported by similar findings of previous works.

Composite development and stratigraphy of the Onepoto maar lake sediment sequence (Auckland Volcanic Field, New Zealand)

The accurate and precise reconstruction of Quaternary climate as well as the events that punctuate it is an important driver of the study of lake sediment archives. However, until recently lake sediment-based palaeoclimate reconstructions have largely concentrated on Northern Hemisphere lake sequences due to a scarcity of continuous and high-resolution lake sediment sequences from the Southern Hemisphere, especially from the southern mid-latitudes. In this context, the deep maar lakes of the Auckland Volcanic Field of northern New Zealand are significant as several contain continuous and well-laminated sediment sequences. Onepoto Basin potentially contains the longest temporal lake sediment record from the Auckland Volcanic Field (AVF), spanning from Marine Isotope Stage 6e (MIS 6e) to the early Holocene when lacustrine sedimentation was terminated by marine breach of the south-western crater tuff ring associated with post-glacial sea-level rise. The Onepoto record consists of two new, overlapping cores spanning ca. 73 m combined with archive material in a complete composite stratigraphy. Tephrochronology and 14C dating provide the fundamental chronological framework for the core, with magnetic relative palaeo-intensity variability downcore, and meteoric 10Be influx into the palaeolake to refine the chronology. The µ-XRF (micro X-ray fluorescence) downcore variability for the entirety of the lake sediment sequence has been established with measurement of a range of proxies for climate currently underway. This work will produce the first continuous record of the last 200 kyr of palaeoclimate from northern New Zealand to date.

Lake Chala turbidites produced by surficial slope sediment remobilization: A mechanism to bring near-shore macrofossils to the deep basin with only limited time offset

&lt;p&gt;In paleoclimate and paleoenvironmental studies, turbidites are usually considered as interruptions of the sedimentary sequence and therefore ignored. However, turbidites are composed of sediments from the (shallow) slopes along the lake&amp;#8217;s periphery where fossil assemblages are often different to those in the deep basin. Turbidites may thus be valuable as carriers of this near-shore proxy information to a profundal core site. However, as turbidites are composed of reworked (older) sediments, their fossil content can only be exploited if their &amp;#8220;mean time offset&amp;#8221; can be readily estimated or ascertained to be minimal.&lt;/p&gt;&lt;p&gt;Several recent studies have shown that turbidites can indeed form as a result of surficial slope sediment remobilization, a process &amp;#8211; independent of slope failure &amp;#8211; in which only a thin veneer (20 cm) of surficial sediment is being remobilized, for example by earthquake shaking, and subsequently transported by a turbidity current. However, demonstrating that this process is active in a basin and determining the remobilization depth, is challenging, especially in the absence of slope cores. Here we study the turbidite record of the 215 m (~260 kyr) long composite core of Lake Chala in the framework of the ICDP project DeepCHALLA. We analyzed its sediment color at a 0.5-cm interval using a spectrophotometer and determined the average color for each of the 391 thickest turbidites (&gt; 3 datapoints) in the L*a*b* color space. For the entire dataset, we performed a linear regression of the turbidite color against the average color of different intervals (2-55 cm) of laminated sediment below. For each combination of paired values, the highest R&amp;#178; values are found for the upper 7-15 cm of matrix sediment below the turbidites, which can thereby be interpreted as the average remobilization depth. These results are mainly based on the a* value, which shows (i) relatively poor correlations between adjacent intervals of laminated sediment (thereby not smearing the signal), and (ii) the most constant values in sediments from across the basin as determined by short-core transects. Depth-dependent variations of sediment color as determined from these transects further allows to estimate the water depth from which the turbidites were sourced.&lt;/p&gt;&lt;p&gt;Our results show that the sediments of most Lake Chala turbidites are 100-200 yrs older than the laminated sediments upon which they are deposited. We conclude that the turbidites can be used as &amp;#8216;sampling windows&amp;#8217; to study temporal trends in macrofossils such as ostracods, chironomids and fish teeth, which are much more common along the basin periphery than in the deep basin.&lt;/p&gt;

Dehydration and stabilization of unconsolidated laminated lake sediments using gypsum for the preparation of thin sections

An improved Lamoureux method for subsampling of unconsolidated laminated sediment is described. Here, we describe a new methodological approach that changes the Lamoureux method in four steps of which the most important change concerns dehydration and stabilization. In this step, we adopted gypsum embedding of the subsample, which took about 1 h to harden and keeps the sediment partially moist. After drying of the gypsum, the subsamples are impregnated with Epoxy 2000 resin under room temperature. This method requires commonly available equipment and can be implemented cost-effectively within 3–4 days.

VARDA (VARved sediments DAtabase) – providing and connecting proxy data from annually laminated lake sediments

Varved lake sediments provide climatic records with seasonal to annual resolution and low associated age uncertainty. Robust and detailed comparison of well-dated and annually laminated sediment records is crucial for reconstructing abrupt and regionally time-transgressive changes as well as validation of spatial and temporal trajectories of past climatic changes. The VARved sediments DAtabase (VARDA) presented here is the first data compilation for varve chronologies and associated palaeoclimatic proxy records. The current version 1.0 allows detailed comparison of published varve records from 95 lakes. VARDA is freely accessible and was created to assess outputs from climate models with high-resolution terrestrial palaeoclimatic proxies. VARDA additionally provides a technical environment that enables us to explore the database of varved lake sediments using a connected data model and can generate a state-of-the-art graphic representation of a multisite comparison. This allows the reassessment of existing chronologies and tephra events to synchronize and compare even distant varved lake records. Furthermore, the present version of VARDA permits the exploration of varve thickness data. In this paper, we report in detail on the data-mining and compilation strategies for the identification of varved lakes and assimilation of high-resolution chronologies, as well as the technical infrastructure of the database. Additional palaeoclimatic proxy data will be provided in forthcoming updates. The VARDA graph database and user interface can be accessed online at https://varve.gfz-potsdam.de (last access: 15 September 2020), all datasets of version 1.0 are available at https://doi.org/10.5880/GFZ.4.3.2019.003 (Ramisch et al., 2019).